Composite Comprising Array of Needle-Like Crystal, Method for Producing the Same, Photovoltaic Conversion Element, Light Emitting Element, and Capacitor

ABSTRACT

A composite of a base and an array of needle-like crystals formed on a surface of the base is provided, in which the base side and the opposite side to the base with respect to the array can be isolated in a satisfactory manner. A composite  10  contains a transparent electrode  2  serving as the base and an array  4  of needle-like crystals  3  formed thereon. The needle-like crystals  3  are made of, for example, zinc oxide. The array  4  includes a first region R 1  on the transparent electrode  2  side and a second region R 2  on the opposite side to the transparent electrode  2  with respect to the first region R 1 . A proportion of the cross section of the needle-like crystals  3  in a plane parallel to the surface of the transparent electrode  2  is lower in the second region R 2  than in the first region R 1 , and the surface of the transparent electrode  2  is substantially covered with the needle-like crystals  3  in the first region R 1.

TECHNICAL FIELD

The present invention relates to a composite comprising an array ofneedle-like crystals, a method for producing the same, and aphotovoltaic conversion element, a light emitting element, and acapacitor using the composite.

BACKGROUND ART

A photovoltaic conversion element, a light emitting element, acapacitor, and the like are elements employing a phenomenon taking placewhen two kinds of materials are brought into contact with each other ordisposed in close proximity to each other. For example, the photovoltaicconversion element and the light emitting element employ a phenomenontaking place on the contact interface between a p-type semiconductor andan n-type semiconductor that light is generated by generation ofelectron-hole pairs or recombination of electrons and holes according toan amount of received light. The capacitor employs polarization of adielectric material sandwiched between a pair of electrodes.

It is possible for these elements to enhance the properties of theelements by increasing the contact area or the opposing area of twokinds of materials. For example, by increasing the area of the interfacebetween the p-type semiconductor and the n-type semiconductor, an amountof electron-hole pairs to be generated, that is, the magnitude of aphotoelectric current (photoelectromotive force) can be increased in thephotovoltaic conversion element, and an amount of light to be generatedcan be increased in the light emitting element. Also, by increasing thecontact (opposing) area of the dielectric material and the electrodes,the electrostatic capacity can be increased in the capacitor.

In order to increase the contact area or the opposing area of two kindsof materials, an attempt is being made by using an aggregate ofneedle-like crystals having a large surface area (specific surface area)for these elements.

For example, Patent Document 1 specified below discloses a photovoltaicconversion element including a transparent electrode, needle-likecrystals forming one charge transporting layers and formed on thetransparent electrode, and the other charge transporting layer providedto come into contact with (to oppose) the needle-like crystals.

Also, Patent Document 2 specified below discloses a capacitor includinga flat storage node made of polycrystalline silicon, plural needle-likecrystals made of a material having the conducting property, such asgermanium, and formed on the storage node, and an insulating film(dielectric material) for capacitor made of silicon oxide and providedto cover the surface of the needle-like crystals.

Patent Document 3 specified below discloses a method for producing anarray of needle-like crystals by heating a substrate (base) in asolution containing amine, such as hexamethylene tetramine, as well aspolyethyleneimine and zinc ions, and a device using the array ofneedle-like crystals produced by this producing method. As such adevice, a dye sensitizing photovoltaic cell (a dye sensitizingphotovoltaic cell using the array of needle-like crystals as asemiconductor having the p-type physical property) and a light emittingdiode are disclosed therein. Patent Document 3 specified below furtherdiscloses a current versus voltage characteristic of an FET (FieldEffect Transistor) using an array of zinc oxide needle-like crystals.

According to the producing method of Patent Document 3 specified below,it is impossible to cover the base with the array densely (substantiallycompletely) in a region on the base side. Accordingly, as means forpreventing current leakage from a portion of the base exposed throughthe array, an electronic block layer (insulator) is provided inclearances among needle-like crystals forming the array when the deviceis fabricated.

Also, Non-Patent Document 3 specified below discloses a method forproducing a light emitting diode using an array of zinc oxideneedle-like crystals and the properties thereof. An array of zinc oxideneedle-like crystals is formed by means of electro-deposition. However,because it is impossible to cover the base with the array densely(substantially completely) in a region on the base side, an insulator isprovided in clearances among needle-like crystals to prevent currentleakage.

In Patent Document 3 and Non-Patent Document 3 specified below,insulating polymer, such as polymethyl methacrylate and polystyrene orthe like, is used as an insulator to prevent the generation of a leakcurrent. The insulator in each is formed to cover the zinc oxideneedle-like crystals first, and thence a portion present at the tip endsof zinc oxide needle-like crystals is removed by means of UVirradiation, plasma irradiation, or the like, so that the insulator isleft in clearances among the zinc oxide needle-like crystals.

Non-Patent Document 4 discloses a method for producing an array of zincoxide needle-like crystals on a foundation layer made of zinc oxide bymeans of electroless plating. According to this method, a layer used asthe foundation layer is obtained by applying a 2-methoxymethanolsolution, in which zinc acetate dihydrate and monoethanolamine aredissolved, on the base followed by drying at 60° C. for 24 hours. Thethickness of the foundation layer is in the order of 100 nm. In a casewhere the concentration of zinc in the plating solution is 0.01 mol/l,an array of zinc oxide needle-like crystals is obtained by regulating apH of the plating solution during plating to 9 to 13.

Non-Patent Document 5 specified below discloses a method in which a zincoxide thin film is formed on a glass substrate by means of sputteringand an array of zinc oxide needle-like crystals is formed using thiszinc oxide thin film as the seeds. It is said that this method makes itpossible to obtain needle-like crystals aligned in orientation incomparison with needle-like crystals obtained by general liquid phasegrowth.

-   -   Patent Document 1: Japanese Unexamined Patent Publication No.        2002-356400    -   Patent Document 2: Japanese Unexamined Patent Publication No.        6-252360    -   Patent Document 3: US-2005-0009224-A1

Non-Patent Document 1: Michael H. Huang and eight others,“Room-Temperature Ultraviolet Nanowire Nanolasers”, SCIENCE vol. 292 p.1897-1899 (8 Jun., 2001)

Non-Patent Document 2: Masanobu Izaki and one other, “Transparent zincoxide films prepared by electrochemical reaction”, Appl. Phys. Lett.68(17), (22 Apr., 1996)

Non-Patent Document 3: R. Konenkamp and two others, “UltravioletElectroluminescence from ZnO/Polymer Heterojunction Light-EmittingDiodes”, Nano Letters, vol. 5 p. 2005 (17 Sep., 2005)

Non-Patent Document 4: Satoshi Yamabi and one other, “Growth conditionsfor wurtzite zinc oxide films in aqueous solutions”, J. Mater. Chem.,12, 3773, (2002)

Non-Patent Document 5: R. B. Peterson and two others, “EpitaxialChemical Deposition of ZnO Nanocolumns from NaOH Solutions”, Langmuir,20, 5114, (2004)

Non-Patent Document 6: edited by the Surface Science Society of Japan,Kaitei-ban, Hyoumen Kagaku no Kiso to Ouyou, NTS Inc.

Non-Patent Document 7: A. Fujishima and one other, “ElectrochemicalPhotolysis of Water at a Semiconductor Electrode” Nature, 238, 37 (1972)

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

In the photovoltaic conversion element of Patent Document 1 supra,however, each needle-like crystal extends almost perpendicularly to thesurface of the transparent electrode (base) and adjacent needle-likecrystals are disposed spaced apart from each other. Hence, thetransparent electrode is not completely covered with an array ofneedle-like crystals and a portion of the transparent electrode exposedthrough the array of needle-like crystals is in contact with the othercharge transporting layer. This causes a leak current to flow betweenthe transparent electrode and the other charge transporting layer, andefficiency as a photovoltaic conversion element is deteriorated.

The structure of this photovoltaic conversion element using needle-likecrystals may be applied to a light emitting element. In such a lightemitting element, however, a leak current flows between the transparentelectrode and the other charge transporting layer as in the case of thephotovoltaic conversion element, and efficiency of the light emittingelement is deteriorated.

In the capacitor of Patent Document 2 supra, too, because eachneedle-like crystal extends almost perpendicularly to the surface of thestorage node and adjacent needle-like crystals are disposed spaced apartfrom each other, the storage node is not completely covered with anarray of needle-like crystals and a portion of the storage node exposedthrough the array of needle-like crystals is in contact with theinsulating film for capacitor. In short, the insulating film forcapacitor is in contact with both the storage node and the needle-likecrystals, which are materials having different electric physicalproperties.

This makes it impossible to apply a voltage homogeneously across theinsulating film for capacitor, and a loss of the electrostatic capacityoccurs in this capacitor. In addition, because an electric double layeris formed at the interface between the storage node and the needle-likecrystals while a voltage is applied to the insulating film forcapacitor, a loss of potential occurs, which gives rise to a loss of thecharge storage.

The structure of this capacitor using needle-like crystals may beapplied to an electric double layer capacitor. In this case, anelectrolytic solution is used instead of the insulating film forcapacitor, and as needle-like crystals, those functioning as apolarizable electrode can be used. In this case, however, theelectrolytic solution and the base corresponding to the storage nodecome into direct contact with each other, too. Because the electrolyticsolution contains a supporting salt to reduce internal resistance, thecontact between the base and the electrolytic solution gives rise to aleak current.

Also, in the devices disclosed in Patent Document 3 supra and Non-PatentDocument 3 supra, the insulator for preventing leakage may cause aninconvenience as it gets colored due to deterioration by the use over along period and deterioration by heat and light, or may cause aninconvenience in properties (for example, electric resistance) due todiffusion of impurities into the device ascribed to the use of theinsulator. Further, the largeness of the effective surface area is theadvantage of using an array of needle-like crystals. Nevertheless, theeffective surface area is reduced by providing insulating polymer inclearances of the array of needle-like crystals.

In addition, according to the producing method of Non-Patent Document 4supra, the presence of a pin hole in the foundation layer makes itimpossible to cover the base exposed from beneath the foundation layervia this pin hole with an array of needle-like crystals. The foundationlayer therefore has to be made thicker so that a pin hole will not beformed in the foundation layer. Making the foundation layer thicker,however, readily gives rise to an inconvenience that the array ofneedle-like crystals separates from the base and a part of the base isexposed.

According to the producing method of Non-Patent Document 5 supra,because the needle-like crystals obtained are those extending in aperpendicular direction to the base in an aligned fashion (a range ofthe angle produced with the base is small), when an attempt is made toincrease the effective surface area of needle-like crystals by securingclearances among needle-like crystals, a problem arises that the base isexposed. Needle-like crystals extend in a perpendicular direction to thebase in an aligned fashion because the zinc oxide thin film formed bymeans of sputtering and forming the foundation layer is an orientationfilm and the crystal orientations in a portion of the foundation layerthat becomes the staring points for growth of needle-like crystals havebeen already aligned.

Such being the case, an object of the invention is to provide acomposite of a base and an array of needle-like crystals formed on thesurface of the base, in which the base side and the opposite side to thebase with respect to the array can be isolated in a satisfactory mannerand a method for producing the same.

Another object of the invention is to provide a composite in which thebase side and the opposite side to the base with respect to the arraycan be isolated in a satisfactory manner without using an insulator anda method for producing the same.

A further object of the invention is to provide a method for producing acomposite in which an array of needle-like crystals hardly separatesfrom the base.

Still another object of the invention is to provide highly efficientphotovoltaic conversion element and light emitting element.

Still another object of the invention is to provide a capacitor capableof enhancing the electrostatic capacity.

Still another object of the invention is to provide a capacitor capableof reducing a leak current.

Means for Solving the Problems

A composite according to a first aspect of the present inventioncomprises a base and an array of a plurality of needle-like crystalsmade of oxide and formed on a surface of the base, wherein the arrayincludes a first region on a side of the base and a second region on anopposite side to the base with respect to the first region, wherein aproportion of a cross section of the needle-like crystals in a planeparallel to the surface of the base is lower in the second region thanin the first region and the surface of the base is substantially coveredwith the needle-like crystals in the first region.

A composite according to a second aspect of the present inventioncomprises a base and an array of a plurality of needle-like crystalsmade of oxide and formed on a surface of the base, wherein the arrayincludes a first region on a side of the base and a second region on anopposite side to the base with respect to the first region, wherein anorientation degree of the needle-like crystals in the first region islow in comparison with the second region and the surface of the base issubstantially covered with the needle-like crystals in the first region.

In these composites, the surface of the base is substantially completelycovered with the needle-like crystals in the first region. In otherwords, the entire surface of the base is covered with the needle-likecrystals. It is therefore possible in this composite to isolate the baseside and the opposite side to the base with respect to the array ofneedle-like crystals in a satisfactory manner.

In the composite according to the first aspect, for example, aproportion of the cross section (area) of the needle-like crystals in aplane parallel to the surface of the base may become lower in the secondregion than in the first region because the number of needle-likecrystals per unit volume is smaller in the second region than in thefirst region.

In the composite according to the second aspect, because the needle-likecrystals in the first region are allowed to extend in variousdirections, they are able to cover the surface of the base efficiently.It is possible to confirm that the orientation degree of the needle-likecrystals in the first region is lower than the orientation degree of theneedle-like crystals in the second region, for example, by separatingthe array from the base and performing the X-ray diffraction analysis ofthe array on the side (first region) where the base was present and onthe opposite side (second region). For example, in one measurementexample, the X-ray diffraction pattern of the second region shows the(002) plane peak and the (100) plane peak alone, whereas the X-raydiffraction pattern of the first region shows the (101) plane peak inaddition to these peaks.

In a case where the composite includes a large number of plural externalneedle-like crystals (described more in detail below) extending from theend portions of the needle-like crystals on the opposite side to thebase to the outside of the array region of the needle-like crystals,there may be a case where it is impossible to confirm a difference ofthe orientation degree between the first region and the second region bythe X-ray diffraction analysis. However, even in such a case, anobservation of an EBSP (Electron Back Scatter Pattern) makes it possibleto confirm that the orientation degree of needle-like crystals is low inthe first region in comparison with the second region. This is because,in contrast to the X-ray diffraction by which an average of all theregions in the measured plane of the array subjected to measurement isobtained as information about the orientation degree of needle-likecrystals, information (plane orientation) for each region within themeasured plane can be obtained from the EBSP.

It should be noted, however, that in a case where the second region sideof the array is covered with the external needle-like crystals almostentirely (for example, in a case where the external needle-like crystalsalone are confirmed when the second region side of the array ofneedle-like crystals is observed using a scanning electron microscope),it is impossible to confirm a difference of the orientation degree ofthe needle-like crystals between the first region and the second regionof the array by the EBSP.

In such a case, a measurement sample may be polished from the side ofthe external needle-like crystals by means of mechanical polishing,chemical mechanical polishing, and the like until the array (secondregion) of the needle-like crystals is exposed, and the first regionside and the second region side of this array are observed by the EBSP.Accordingly, it becomes possible to confirm the orientation degree ofthe needle-like crystals on each of the first region side and the secondregion side of the array. In this case, it is preferable to fillclearances of the array of needle-like crystals with resin or the likebefore the measurement sample is polished.

A composite according to a third aspect of the present inventioncomprises a base and an array of a plurality of needle-like crystalsmade of oxide and formed on a surface of the base, wherein theneedle-like crystals include first needle-like crystals extending fromrespective plural starting points positioned spaced apart from oneanother on the surface of the base in a direction producing an anglewith the surface of the base such that falls within a specific angularrange, and second needle-like crystals extending from the respectivestarting points in a direction within a wider angular range encompassingthe specific angular range and shorter than the first needle-likecrystals, wherein a portion on the surface of the base exposed throughthe first needle-like crystals is substantially covered with the secondneedle-like crystals.

In this composite, a portion on the surface of the base exposed throughthe first needle-like crystals is substantially (completely) coveredwith the second needle-like crystals. In other words, the surface of thebase is substantially completely covered with the needle-like crystals(first and second needle-like crystals). It is thus possible in thecomposite to isolate the base side and the opposite side to the basewith respect to the array of needle-like crystals in a satisfactorymanner.

In any of the composites according to the first through third aspects ofthe invention, because the base side and the opposite side to the baseare isolated in a satisfactory manner virtually by the needle-likecrystals alone, there is no need to cover the base with an insulatorauxiliary for this isolation. In short, in the composites of theinvention, the base side and the opposite side to the base with respectto the array can be isolated in a satisfactory manner without using aninsulator.

Because the need to provide an insulator in a space among theneedle-like crystals is eliminated, it is possible to increase theeffective surface area of the needle-like crystals.

In addition, because there is no need for these composites to use aninsulator for isolating the base side and the opposite side to the basewith respect to the array in a satisfactory manner, in a device usingthese composites, neither an inconvenience resulting from deteriorationor coloring of the insulator nor diffusion of impurities into the deviceascribed to the use of the insulator will occur.

The array referred to herein is defined as an aggregate of needle-likecrystals for which the frequency of angles produced between the surfaceof the base and the respective needle-like crystals becomes higher at aparticular angle (for example, 90°). The specific angular range (adistribution range of the angle produced between the surface of the baseand the longitudinal direction of the first needle-like crystals) is,for example, 45° to 90°. A distribution range of the angle producedbetween the surface of the base and the longitudinal direction of thesecond needle-like crystals may be, for example, 0° (parallel to thesurface of the base) to 90° (perpendicular to the surface of the base).

The needle-like crystals referred to herein mean so-called whiskershaving an aspect ratio of crystals of, for example, 5 or higher. Theaspect ratio of the needle-like crystals is preferably 10 or higher, andmore preferably, 100 or higher. Also, the minimal length passing thecenter of gravity in the traverse cross section (cross sectionperpendicular to the longitudinal direction) of the needle-like crystalsis preferably 500 nm or smaller, more preferably, 100 nm or smaller, andfurther preferably, 50 nm or smaller.

With the composite as above, the needle-like crystals can be used as anelectrode in batteries (a device that chemically stores energy, such asa lithium-ion battery and a fuel cell) and an electron source in anelectron emitting device. When used as the foregoing, it is preferablethat the needle-like crystals have a higher aspect ratio (for example,preferably 10 or higher).

The needle-like crystals include defect-free acicular single crystalsand those having screw dislocation, and they may have polycrystals or anamorphous portion. The outer shape of the needle-like crystals includesbut not limited to a circular column, a circular cone, and a hexagonalcolumn, or a circular cone with a flat tip end (truncated cone) and acircular column with a pointed tip end or a flat tip end. Further, theouter shape of the needle-like crystals includes but not limited to atriangular pyramid, a square pyramid, a hexagonal pyramid, a polygonalpyramid other than the foregoing, and a polygonal pyramid with a flattip end, and a triangular column, a square column, a hexagonal column,and a polygonal column other than the foregoing, or a triangular column,a square column, a hexagonal column, and a polygonal column other thanthe foregoing with a pointed tip end or a flat tip end. The outer shapemay further include a bent form of the foregoing.

The oxide may be a compound made of at least one kind of element amongZn, Ti, Zr, Hf, Ni, Fe, Co, Na, K, Li, Mg, Ca, Sr, Ba, Si, Al, Ga, In,V, Nb, Ta, W, Mo, Cr, and Sn and oxygen, or it may include at least onekind of element among B, C, N, S, P, F, and Cl as a dopant or an elementthat substitutes for oxygen in part.

The base referred to herein is a substance that holds the needle-likecrystals (array) at least after the needle-like crystals are grown. Thesurface of the base may be a flat surface, a concavo-convex surface, aspherical surface, and so forth. A constituting material, a thickness, ashape, and an optical property of the base can be designed as needed tosuit the required durability and the design. For example, a glasssubstrate, a glass substrate on which a transparent conductive film isapplied, plastic, paper given with the water resistance, ceramic, ametal plate, a shaped metal plate, and so forth can be used as needed.

The specific direction and the c-axis of the needle-like crystals maycoincide with each other, and in this case, the longitudinal directionof the needle-like crystals may be oriented in the specific direction.In this case, the c-axes of the needle-like crystals are oriented withrespect to the specific direction.

In this case, when the crystal system of the needle-like crystals is,for example, the hexagonal system, the optical property in the in-planedirection perpendicular to the specific direction is homogeneous atleast in the second region. This eliminates a variance of the opticalproperty resulting from the crystal orientation, such as a degree ofdouble refraction, and for example, when this composite is applied to alight emitting element, it becomes possible to extract homogeneouslight. In addition, in this case, the electric property also becomeshomogeneous in the second region in the in-plane direction perpendicularto the specific direction. Hence, when this composite is applied to alight emitting element, it is possible to reduce irregularities of lightemission and when this composite is applied to a photovoltaic conversionelement, it is possible to stabilize the conversion efficiency.

The composite may further comprise a plurality of external needle-likecrystals extending from end portions of the needle-like crystals on anopposite side to the base to an outside of an array region of theneedle-like crystals.

For example, in a case where this composite is used for a photovoltaicconversion element while the base is a transparent electrode and theneedle-like crystals are a part of a p-n junction body, it is possibleto achieve photovoltaic conversion by allowing light to go incident onthe p-n junction portion from the base side. In this case, thephotovoltaic conversion efficiency can be increased by reflecting(scatting) light having passed through the array region of theneedle-like crystals once by the external needle-like crystals andintroducing the light to the p-n junction portion.

The plurality of external needle-like crystals may extend in randomdirections within an angular range within which no interference with thearray occurs. More specifically, the external needle-like crystals maybe grown in plural axial directions in the outside of the array regionof the needle-like crystals. In this case, for example, it is possibleto reflect light having passed through the array region of needle-likecrystals efficiently toward the p-n junction portion by the externalneedle-like crystals in the application example of the photoelectricelement described above. In addition, by providing the externalneedle-like crystals, it is possible to increase the specific surfacearea of the composite.

The needle-like crystals may be made of zinc oxide.

A plurality of microscopic regions having random crystal orientationsmay be present in a vicinity of an interface between the array and thebase.

When needle-like crystals are grown, needle-like crystals may be grownfrom particles that function as seeds. In this case, at least a part ofthe particles that function as the seeds may possibly remain as themicroscopic regions after the growth of needle-like crystals completes.The adhesion between the needle-like crystals and the base can beenhanced by the microscopic regions. The microscopic regions may be madeof the same material as the needle-like crystals or they man by made ofa different material. Alternatively, the microscopic regions may be madeof a fluorescent substance or a conductive material having a suitableelectric conductivity. In a case where these materials are used, it ispossible to confer a suitable optical or electric property to thecomposite.

A photovoltaic conversion element according to the present inventionincludes the composite described above. The array in the composite is ofone conduction type, and the photovoltaic conversion element furtherincludes a semiconductor portion of an opposite conduction type opposinga surface of the needle-like crystals.

One conduction type and the opposite conduction type mean one and theother one of the p-type and the n-type, respectively. This photovoltaicconversion element is able to perform photovoltaic conversion by the p-njunction of the array of the one conduction type and the semiconductorportion of the opposite conduction type. Because the surface of the baseis substantially completely covered with the array (first and secondneedle-like crystals), the semiconductor portion of the oppositeconduction type and the base are isolated in a satisfactory manner,which in turn reduces the contact area therebetween. It is thus possibleto reduce a leak current between the semiconductor portion of theopposite conduction type and the base. The conversion efficiency can betherefore increased.

The semiconductor portion of the opposite conduction type is a portionwhere it has virtually the opposite conduction type for the array of theone conduction type, and it may be a conductive material that forms theSchottky junction with the semiconductor portion of the one conductiontype. In this case, for one of the p-type and the n-type, which is theconduction type (the one conduction type) of the semiconductor portion,the conductive material is able to conduct a current as the otherconduction type.

The photovoltaic conversion element includes but not limited to anorganic EL element, a photovoltaic cell (solar battery), and aphotodetector. The base may be a metal electrode or it may be made of asemiconductor of the one conduction type (the same conduction type asthe array).

This photovoltaic conversion element may be a so-called dye sensitizingphotovoltaic conversion element (photovoltaic cell), in which a dyeserving as a light absorption layer is provided between the array of theone conduction type and the semiconductor portion of the oppositeconduction type (in the vicinity of the interface).

The photovoltaic conversion element that may contain a photocatalystconverts light energy to electric energy and that accelerates a chemicalreaction using the electric energy.

By using electric energy stored by light excitation for a chemicalreaction taking place on the surface of the photocatalyst, thephotocatalyst is able to accelerate decomposition of, for example,organic matter, water or the like. A photocatalytic reaction isclassified into a photoelectrochemical reaction. As an applicationexample of the photovoltaic conversion element, there is an element(device) that gives rise to a photocatalytic reaction (see Non-PatentDocument 7 supra).

The array of needle-like crystals of the invention may be made of aphotocatalyst (may contain a photocatalyst). In this case, the array canbe used as a photocatalytic electrode. Because the array of needle-likecrystals of the invention has a large surface area, a reactionefficiency of the needle-like crystals (array) made of (containing) aphotocatalyst is high

In a case where the array of needle-like crystals of the invention isused as a photocatalytic electrode, the array accelerates a chemicalreaction (for example, an oxidative reaction) via one of electrons andholes for the other one of electrons and holes to be transported fromthe base, which can in turn accelerate a different reaction (forexample, a reductive reaction) in another system. Another systemreferred to herein may be, for example, a system including a counterelectrode (for example, a counter electrode made of platinum)electrically connected to the base.

Conventionally, a photocatalyst in the form of fine particles is used toincrease the area of the surface of the photocatalyst, which is areaction field. The photocatalyst in the form of fine particles isdispersed in a system and irradiated by light. When configured in thismanner, it is possible to increase the effective surface area. It shouldbe noted, however, that because electrons and holes that respectivelyaccelerate different reactions are present within the samephotocatalyst, recombination of electrons and holes is readilyaccelerated. Also, in this system, a contact among the photocatalyst canoccur and recombination of electrons and holes is readily acceleratedalso by such a contact.

In the composite of the invention, because the plurality of needle-likecrystals can be brought into an electrically connected state, it ispossible to give rise to reactions via one and the other of electronsand holes, respectively, in a system including the array and anothersystem as described above. In other words, because chemical reactionscan be accelerated while reaction fields of electrons and holes areseparated, recombination of electrons and holes hardly takes place. Inaddition, because the needle-like crystals substantially cover the base,a probability is low for electrons or holes transported to the base torecombine with holes or electrons.

Conventionally, a photocatalyst made of a semiconductor and in the formof a flat plate is used as the electrode in some cases. Even for such aphotocatalyst forming an electrode in the form of a flat plate, it ispossible to reduce a probability of the recombination of electrons andholes. The reaction efficiency, however, is low, because the effectivesurface area cannot be increased due to the electrode shape.

In the invention of the present application, in a case where the arraycontains a photocatalyst, not only is it possible to increase thereaction efficiency by increasing the effective surface area, but it isalso possible to reduce the recombination of electrons and holes.

When used as a photocatalyst, it is possible to adjust an absorption endwavelength by subjecting the array to doping or substituting oxygen inthe array with nitrogen or the like in part. For example, by dopinggallium into a part of the array of zinc oxide followed by calcinationin an ammonia atmosphere, it is possible to nitride doped gallium(substitute oxygen in the array of zinc oxide with nitrogen). It hasbeen revealed that such a material is capable of decomposing water usinglight energy in a visible light region.

A light emitting element according to the present invention includes thecomposite descried above. The array in the composite is of oneconduction type, and the light emitting element further includes asemiconductor portion of an opposite conduction type opposing a surfaceof the needle-like crystals.

The light emitting element includes but not limited to a semiconductorlight emitting element (photodiode) and a semiconductor laser. The basemay be a metal electrode or it may be made of a semiconductor of the oneconduction type (the same conduction type as the array).

By applying a suitable voltage to the p-n junction portion of the arrayof the one conduction type and the semiconductor portion of the oppositeconduction type, the light emitting element is able to generate light byrecombination of electrons and holes. Because the surface of the base issubstantially completely covered with the array, the semiconductorportion of the opposite conduction type and the base are isolated in asatisfactory manner, which in turn reduces the contact areatherebetween. It is thus possible to reduce a leak current between thesemiconductor portion of the opposite conduction type and the base. Thelight emitting efficiency can be therefore increased.

In the photovoltaic conversion element and the light emitting element ofthe invention, the array of the one conduction type and thesemiconductor portion of the opposite conduction type may function as alight receiving material (in the case of the photovoltaic conversionelement) and a light emitting material (in the case of the lightemitting element) The photovoltaic conversion element and the lightemitting element of the invention may include a non-doped lightreceiving or light emitting material or a light receiving or lightemitting material doped for the purpose of changing the opticalproperty, for example, between the array of the one conduction type andthe semiconductor portion of the opposite conduction type.

The light receiving or light emitting material referred to herein meansa material that converts light to electricity or a material thatconverts electricity to light by utilizing a band gap, and an organicdye, a pigment, a fluorescent substance, and a semiconductor are usedfor the material. As long as the photoelectric converting or lightemitting function is exerted, the morphology as to whether it is acrystalline, amorphous, or monomolecular material or an aggregate is notparticularly specified.

A capacitor according to still another aspect of the present inventionincludes the composite described above. The array in the compositefunctions as a first electrode, and the capacitor further includes asecond electrode opposing the first electrode and a dielectric materialinterposed between the first electrode and the second electrode.

The capacitor referred to herein means a device that physically storescharges.

Because the surface of the base is substantially completely covered withthe array, the dielectric material and the base are isolated in asatisfactory manner, which in turn reduces the contact areatherebetween. More specifically, the dielectric material is in contactwith virtually the first electrode (array) alone on the opposite side tothe second electrode. Hence, in a case where the needle-like crystalsare made of a single kind of material (oxide), it is possible to apply avoltage to the dielectric material homogeneously. In addition, anelectric double layer will not be formed between the base and the array.The capacitor is thus able to enhance the electrostatic capacity.

A capacitor according to still another aspect of the present inventionincludes the composite described above. The array in the compositefunctions as a first polarizable electrode, and the capacitor furtherincludes a second polarizable electrode opposing the first polarizableelectrode and an electrolytic solution interposed between the firstpolarizable electrode and the second polarizable electrode.

This capacitor may be a so-called electric double layer capacitor inwhich an electric double layer is formed in the vicinity of theinterfaces between the respective first and second polarizableelectrodes and the electrolytic solution. Because the surface of thebase is substantially completely covered with the array, theelectrolytic solution and the base are isolated in a satisfactorymanner, which in turn reduces the contact area therebetween. Hence, evenin a case where the base forms a part of the electrode, it is possibleto reduce a leak current after the capacitor is fully charged, which canin turn avoid wasteful power consumption.

In an electric double layer capacitor configured to respond to quickcharging, the electrolytic solution contains the electrolyte excessivelyand the electrolyte is present in the electrolytic solution even in afully charged state. When the electrolytic solution is in contact withthe base serving as one of the electrodes, power is consumed wastefullyas a current (leak current) flows between the both electrodes via theelectrolyte remaining after the capacitor is fully charged. On thecontrary, in a case where the electrolytic solution is not in contactwith the base (electrode) as in the capacitor of the present invention,it is possible to suppress such a current. It goes without saying thatit is possible to suppress inverse electron migration, which is wastefulpower consumption, even in a state where no voltage is applied (duringdischarging) after the capacitor is charged.

The electrolytic solution (battery electrolyte) may be either a solutionbased on a non-aqueous solvent or an aqueous solution. The electrolyticsolution based on a non-aqueous solvent is made by dissolving anelectrolyte in an organic solvent, and examples of an available organicsolvent include but not limited to ethylene carbonate, propylenecarbonate, 1-butylene carbonate, sulfolane, γ-butyrolactone,dimethylsulfoxide, dimethylformamide, acetonitrile, tetrahydrofuran, anddimethoxyethane. A mixture of two or more kinds of the foregoing can beused as well.

An available electrolyte for the electrolytic solution based on anon-aqueous solvent includes but not limited to (C₂H₅)₄PBF₄,(C₃H₇)₄PBF₄, (C₂H₅)₄NBF₄, (C₃H₇)₄NBF₄, (C₂H₅)₄PPF₆, (C₂H₅)₄PCF₃SO₃,LiBF₄, LiC₁₋₄, LiCF₃, and SO₃. An available electrolyte for the aqueouselectrolytic solution includes but not limited to NaCl, NaOH, HCl,H₂SO₄, and Li₂SO₄. A polymer electrolytic solution made by adding ahigh-molecular substance to the foregoing can be used as well.

A method for producing the composite according to still another aspectof the invention includes a step of forming the needle-like crystals ona foundation made of a plurality of crystal grains that are crystalgrains having random crystal orientations by means of electrolessplating using a plating solution containing at least one kind of alkaliselected from the group consisting of X¹OH, where X¹ is one of Na, K,and Cs, X² ₂CO₃, where X² is one of H, Na, K, and Cs, and NH₃ and havinga pH of 13 or higher.

A method for producing the composite according to still another aspectof the present invention includes a step of forming the needle-likecrystals on a foundation having a hydrophilic surface and beingamorphous by means of electroless plating using a plating solutioncontaining at least one kind of alkali selected from the groupconsisting of X¹OH, where X¹ is one of Na, K, and Cs, X² ₂CO₃, where X²is one of H, Na, K, and Cs, and NH₃ and having a pH of 13 or higher.

The needle-like crystals may be formed on a foundation made of resinhaving a hydrophilic surface by means of electroless plating using aplating solution containing at least one kind of alkali selected fromthe group consisting of X¹OH, where X¹ is one of Na, K, and Cs, X² ₂CO₃,where X² is one of H, Na, K, and Cs, and NH₃ and having a pH of 13 orhigher.

By allowing the alkali specified above to ionize or by ionizing thealkali in a plating solution at room temperature, it is possible toobtain a plating solution in which hydroxide ions are present.

It is possible to produce the composite described above by the producingmethods of the invention of the present application. In other words,according to the producing method, not only is it possible to form aplurality of needle-like crystals that are grown from the startingpoints on the foundation at various angles with respect to the surfaceof the base, but it is also possible to make the starting points forgrowth on the foundation denser. Also, according to the producingmethod, it is possible to let a large number of needle-like crystals begrown from a single starting point.

On the contrary, in a case where an alkali specified as above as analkali in the plating solution is not used, and/or in a case where aplating solution having a pH smaller than 13 is used, a manner in whichthe needle-like crystals are grown differs significantly. Because thestarting points for growth on the foundation become rougher or thedirections of growth are aligned, it becomes impossible to cover thebase substantially completely with the needle-like crystals (array).

Also, in a case where a foundation that satisfies the specificrequirements specified above is not used, for example, in a case where acrystalline foundation that is a foundation whose crystal orientationsare aligned in the respective portions thereof is used, the directionsof growth of needle-like crystals are aligned, which also makes itimpossible to cover the base with the needle-like crystals (array)substantially completely.

Of the needle-like crystals, those extend in a direction producing anangle with the surface of the base such that falls within a specificangular range (for example, those producing an angle closer to the rightangles with the surface of the base) are grown long and become the firstneedle-like crystals. Meanwhile, of the needle-like crystals, thoseproducing an angle with the surface of the base such that falls outsidethe specific angular range (for example, those producing an angle farfrom the right angles with the surface of the base) and a part of thosewithin the specific angular range cannot be grown long as they arehindered by needle-like crystals grown from another adjacent startingpoint, and they become the second needle-like crystals that cover aportion on the surface of the base exposed through the first needle-likecrystals substantially completely.

With the use of the plating solution specified as above, needle-likecrystals can be grown using a substance having a hydroxyl group on thesurface thereof as a catalyst. A substance having a hydroxyl group onthe surface referred to herein means a substance containing an OH-groupin the structure formula or a substance that forms an OH-group (surfacehydroxyl group) on the surface upon contact with water or the like. Anavailable substance includes but not limited to an inorganic compoundmade of at least one kind of element among Zn, Ti, Zr, Hf, Ni, Fe, Co,Na, K, Li, Mg, Ca, Sr, Ba, Si, Al, Ga, In, V, Nb, Ta, W, Mo, Cr, Eu, Y,La, Gd, Tb, Ce, Nd, Sm, Rb, and Cs and oxygen, and an organic compoundsuch as a polyvinyl alcohol and copolymer of a polyvinyl alcohol. In acase where a substance having a hydroxyl group on the surface is made ofan inorganic compound, the inorganic compound may contain at least onekind of element among B, C, N, S, P, F, and Cl as a dopant. A substancehaving a hydroxyl group on the surface can be either a crystalline oramorphous substance.

Electroless plating referred to herein means a method of plating atarget to be precipitated on a surface by proceeding a reaction not byan electrolytic process but chemically and/or thermally, and it is amethod to proceed plating by impregnating a base with a plating solution(by dipping the former in the latter) and setting the temperature of theplating solution to a specific temperature.

The plating solution referred to herein means a solution that containswater or an organic solvent or a mixture thereof as a solvent and atleast one kind of raw material of a substance to be plated as a soluteand accelerates electroless plating with an additive in the solution. Inthe invention of the present application, an additive contains at leastan alkali to adjust a pH value of the plating solution.

The surface hydroxyl group referred to herein means a hydroxyl grouppresent on the surface of the substance by adsorption of a watermolecule or the like. It is known to function not only as an adsorptionsite, but also as various reaction sites for halogenation,esterification, salt purification, and generation of silanolate (seeNon-Patent Document 6 supra). A surface hydroxyl group, such as oxideand oxynitride, and a surface hydroxyl group resulting from an OH-group,a ketone group, and a carboxyl group, are present, and these are thoughtto function as a reaction site or an adsorption site.

The organic solvent includes an aromatic series, such as ethanol,methanol, isopropanol, butanol, acetylacetone, acetonitrile, ethyleneglycol, and benzene.

A raw material of the substance to be plated is a metal compound, andexamples of which include but not limited to metal nitrate, metalacetate, metal hydrochloride, metal oxalate, metal alkoxide, metalcarbonate, metal sulfate, and at least one kind of salt of metalhydroxide. The metal can be any of Zn, Ti, Zr, Hf, Ni, Fe, Co, Na, K,Li, Mg, Ca, Sr, Ba, Si, Al, Ca, In, V, Nb, Ta, W, Mo, Cr, Eu, Y, La, Gd,Tb, Ce, Nd, Sm, Rb, and Cs. A raw material of the substance to be platedmay be in hydrated form to enable the use of a stable substance.

Examples of an additive in the solution referred herein include but notlimited to metal hydroxide, metal carbonate hydroxide, metal chloride,metal nitrate, metal acetate, metal carbonate, metal alkoxide, metalsulfate, metal oxalate, and surface active agent. Metal in the additivecan be any of Zn, Ti, Zr, Hf, Ni, Fe, Co, Na, K, Li, Mg, Ca, Sr, Ba, Si,Al, Ga, In, V, Nb, Ta, W, Mo, Cr, Eu, Y, La, Gd, Tb, Ce, Nd, Sm, Rb, andCs. The surface active agent referred to herein is a surface activeagent that can be classified into any one of non-ionic, cationic, andanionic active agents as well as an amphoteric active agent having bothcations and anions. By performing electroless plating with an additionof such a surface active agent to the plating solution, it is possibleto achieve an advantage of making the diameter of needle-like crystalsmade of zinc oxide smaller and the aspect ratio thereof higher.

The foundation may contain a surface portion of the base.

Also, the producing methods of the invention may further include a stepof placing particles on the surface of the base. In this case, thefoundation may contain the particles placed on the base. In this case,needle-like crystals are grown using the particles as the seeds.

In a case where the surface of the base does not satisfy therequirements of the foundation as specified above, that is, in a casewhere it satisfies neither of the requirement that it is made of aplurality of crystal grains that are crystal grains having randomcrystal orientations and the requirement that it has a hydrophilicsurface and is amorphous, when needle-like crystals are grown directlyfrom the surface of the base, the crystal orientations of theneedle-like crystals, that is, the directions of growth, are aligned insome cases.

Even in such a case, when the particles placed on the base satisfy therequirements of the foundation, it is possible to randomize the crystalorientations of needle-like crystals grown from the particles (asubstance having a surface hydroxyl group). Because the needle-likecrystals grown randomly become a three-dimension hindrance to another,they bond one to another, which consequently makes it possible to coverthe base substantially with the array of needle-like crystals in thefirst region.

The step of placing the particles may include a step of preparing acolloidal solution by dispersing the particles (fine particles) in asolution and by applying the colloidal solution to the base by means ofdip coating, spray coating, spin coating or dropping. Further, it mayinclude a step of drying and/or calcining when necessity arises.

According to these methods, particles showing the crystal property andhaving random crystal orientations or amorphous particles can be readilyplaced on the surface of the base.

The particles may be made of a same material as the microscopic regions.

The step of placing the particles may include a step of forming a thinfilm made of the particles to have an average thickness of 50 nm orsmaller on the base.

When configured in this manner, it is possible to make the needle-likecrystals (array) hardly separate from the base.

The step of placing the particles may include a step of forming a thinfilm made of the particles to have an average thickness of 20 nm orsmaller on the base.

When configured in this manner, it is possible to make the needle-likecrystals (array) hardly separate from the base in a more reliablemanner.

An average film thickness of the thin film can be smaller, for example,as the particles are taken into the needle-like crystals while theneedle-like crystals are grown. The average thickness of the thin filmat the beginning of the growth of needle-like crystals is preferably 50nm or smaller, and more preferably, 20 nm or smaller.

The thin film made of the particles is a thin film in which theparticles are present homogeneously on the substrate, and it may havevoids or asperities on the surface.

In a case where the plating solution contains the alkali at a 0.001 molconcentration to 2 mol concentration, even when the thickness of thethin film made of the particles that become starting points for growthis small, it is possible to obtain an array of needle-like crystals thatcovers the base substantially completely. When the pH of the platingsolution is 13 or higher as in the invention of the present application,even in the presence of a pin hole in the thin film, it is possible tocover the base with needle-like crystals made of zinc oxidesubstantially completely by increasing the concentration of zinc in theplating solution.

Although the mechanism that gives rise to such a phenomenon is notclear, it is thought that the electrical property and heat conductivityof the plating solution, the catalytic activity of the surface hydroxylgroup, the acidity of a hydroxyl group, the surface energy, and so forthchange with the concentration of alkali, and the growing rate ofneedle-like crystals, the wetting property to the base, a crystalautomorphic shape, and so forth vary with such a change.

The step of placing the particles may include a step of placing aprecursor of a substance forming the particles on the base, and a stepof forming the particles by decomposing the precursor.

For example, the step of placing the particles may include a step ofdispersing a precursor of a desired compound forming the particles in asolvent and applying the solvent to the base by means of dip coating,spray coating, spin coating or dropping, and a step of placing thedesired particles on the base by thermally decomposing the precursor bymeans of drying and/or calcination.

Examples of decomposition include but not limited to decomposition byheat decomposition, hydrolysis, dehydrocondensation, and plasmairradiation.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the invention will be described in detailwith reference to the accompanying drawings.

FIG. 1 is a schematic cross section of a photovoltaic conversion elementaccording to a first embodiment of the invention.

The photovoltaic conversion element 1 comprises a composite 10 includinga transparent electrode 2 serving as the base and an array 4 of aplurality of needle-like crystals 3 formed on the transparent electrode2. The transparent electrode 2 is made of, for example, tin oxide (SnO₂)doped with fluoride (F), and provided, for example, in the form of aflat film on a glass substrate.

The needle-like crystals 3 include first needle-like crystals 3Aextending from respective plural starting points S positioned spacedapart from one another on the surface of the transparent electrode 2 atan angle of almost 90° with respect to the surface of the transparentelectrode 2 (extending perpendicularly to the surface of the transparentelectrode 2), and second needle-like crystals 3B extending from therespective starting points S in directions within a wider angular rangeencompassing a direction perpendicular to the surface of the transparentelectrode 2 (for example, the angle produced with the surface of thetransparent electrode 2 is 0° to 90°) and shorter than the firstneedle-like crystals 3A. In the first and second needle-like crystals 3Aand 3B, the longitudinal directions and the c-axes coincide with eachother. The first needle-like crystals 3A have almost the same length.

One or more than one first needle-like crystal 3A and one or more thanone second needle-like crystal 3B extend from a single starting point S.A clearance is present between adjacent two first needle-like crystals3A. The tip end (the opposite side to the transparent electrode 2) ofthe second needle-like crystal 3B extending from a given starting pointS abuts on or comes in close proximity to the side surface of the firstneedle-like crystal 3A or the second needle-like crystal 3B extendingfrom another starting point S adjacent to the given starting point S.

A portion on the surface of the transparent electrode 2 exposed throughthe first needle-like crystals 3A is substantially (completely) coveredwith the second needle-like crystals 3B. In short, the surface of thetransparent electrode 2 is covered with the needle-like crystals 3(first and second needle-like crystals 3A and 3B) almost completely. Inother words, the entire surface of the transparent electrode 2 iscovered with the needle-like crystals 3. Hence, the transparentelectrode 2 and the opposite side to the transparent electrode 2 withrespect to the array 4 are isolated in a satisfactory manner.

The second needle-like crystals 3B are present in a region (hereinafter,referred as the “first region”) R1 in the vicinity of the surface of thetransparent electrode 2 with respect to a direction perpendicular to thesurface of the transparent electrode 2, and absent in a region(hereinafter, referred to as the “second region”) R2 on the tip end sideof the first needle-like crystals 3A within an array region of theneedle-like crystals 3. Hence, a proportion of the cross section (area)of the needle-like crystals 3 in a plane parallel to the surface of thetransparent electrode 2 is lower in the second region R2 than in thefirst region R1. The orientation degree of the needle-like crystals 3 islow in the first range R1 in comparison with the second range R2. Thec-axes of needle-like crystals 3 as a whole (at least in the secondregion R2) are oriented one another (the frequency at which the c-axesare aligned along a particular direction (a direction perpendicular tothe surface of the transparent electrode 2) is high).

The needle-like crystals 3 are made of, for example, gallium-doped zincoxide and are n-type semiconductors.

A plurality of microscopic regions 7 made of crystals of zinc oxide arepresent between the transparent electrode 2 and the array 4 (chiefly inthe vicinity of the starting points S). The crystal orientations of therespective microscopic regions 7 are random and do not necessarilycoincide with the crystal orientations of the needle-like crystals 3.

A counter electrode 6 is provided to oppose almost in parallel to thetransparent electrode 2 while being slightly spaced apart from the array4 (the tip ends of the first needle-like crystals 3A). The counterelectrode 6 is, for example, formed of a laminate electrode of nickeland gold.

A p-type semiconductor portion 5 is provided to fill a space between thearray 4 and the counter electrode 6, the space including a clearancebetween adjacent first needle-like crystals 3A and a clearance betweenthe tip ends of the first needle-like crystals 3A and the counterelectrode 6. The p-type semiconductor portion 5 is made of, for example,nitrogen-doped zinc oxide or tin sulfide. Because the surface of thetransparent electrode 2 is covered with the first and second needle-likecrystals 3A and 3B almost completely, the contact area of the p-typesemiconductor portion 5 and the transparent electrode 2 is reduced.

A p-n junction is formed by the needle-like crystals 3, which are n-typesemiconductors, and the p-type semiconductor portion 5. Dye particles 8,which are a light absorbing material, are provided between theneedle-like crystals 3 (n-type semiconductor) and the p-typesemiconductor portion 5 (in the vicinity of the interface thereof).

In the photovoltaic conversion element 1, when light comes incident onthe p-n junction portion from the transparent electrode 2 side,electron-hole pairs are generated at the p-n junction portion, and holesand electrons migrate to the counter electrode 6 and the transparentelectrode 2, respectively. This gives rise to a photoelectric current(photoelectromotive force). In this instance, the dye particles 8function to assist an active layer (the needle-like crystals 3 and thep-type semiconductor portion 5) for light absorption. It is thereforepossible to absorb light introduced inside the element efficiently.

Owing to the use of the needle-like crystals 3, the actual surface areaof the first and second needle-like crystals 3A and 3B for the projectedarea onto the base (transparent electrode 2) is large. Hence, the areaof the interface between the needle-like crystals 3 (n-typesemiconductor) and the p-type semiconductor portion 5 is also large,which makes it possible to obtain a large photoelectric current(photoelectromotive force). In this case, the needle-like crystals 3(n-type semiconductor) and the p-type semiconductor portion 5 functionas a light receiving material.

In addition, because the contact area of the p-type semiconductorportion 5 and the transparent electrode 2 is reduced, inverse electronmigration and a leak current are reduced. The conversion efficiency ofthe photovoltaic conversion element 1 is therefore high.

Further, because the first needle-like crystals 3A are almostperpendicular to the surface of the transparent electrode 2 and thelongitudinal direction of the first needle-like crystals 3A coincideswith the c-axis thereof, in a case where the crystal system of theneedle-like crystals 3 is, for example, the hexagonal system, theoptical and electrochemical properties in the in-plane directionparallel to the surface of the transparent electrode 2 are homogeneousin the second region R2. The optical transparency of the composite 10with respect to a direction orthogonal to the surface of the transparentelectrode 2 therefore is higher.

The microscopic regions 7 may function as particles capable ofscattering light. In this case, the photovoltaic conversion efficiencyby the light trapping effect can be enhanced owning to the lightscattering effect inside the element. The microscopic regions 7 may beconductive fine particles. In this case, it is possible to support theelectrical junction between the transparent electrode 2 and the array 4.Further, the microscopic regions 7 may be fluorescent particlesfurnished with the wavelength conversion function. In this case, thephotovoltaic conversion element 1 is able to absorb light having thewavelength out of the light absorption sensitivity of the photovoltaicconversion element 1 as such light undergoes wavelength conversion inthe microscopic regions 7 (fluorescent particles), which can in turnenhance the photovoltaic conversion efficiency. The microscopic regions7, however, may be omitted.

The structure of this photovoltaic conversion element 1 can be appliedto a light emitting element. In this case, by applying a suitablevoltage to the p-n junction portion formed by the array 4 of the n-typeand the p-type semiconductor portion 5 through the transparent electrode2 and the counter electrode 6, it is possible to generate light byrecombination of electrons and holes. In this instance, because theneedle-like crystals 3 are oriented with respect to the c-axes thereof,it is possible to inject electrons into the respective needle-likecrystals 3 homogeneously.

In addition, in such a light emitting element, because the surface ofthe transparent electrode 2 is substantially completely covered with thearray 4, the p-type semiconductor portion 5 and the transparentelectrode 2 are isolated in a satisfactory manner and will not come intocontact with each other. It is thus possible to reduce a leak currentbetween the p-type semiconductor portion 5 and the transparent electrode2, which can in turn increase the light emitting efficiency.

In the case of a light emitting element, when the microscopic regions 7function as particles capable of scattering light, it is possible, forexample, to convert an angle of light incident on the interface betweenthe element and air owing to the light scattering effect inside theelement. The external quantum efficiency is therefore enhanced.

The transparent electrode 2 serving as the base and the opposite side tothe transparent electrode 2 with respect to the array 4 are isolated ina satisfactory manner virtually by the array 4 (needle-like crystals 3)alone, and no insulator for this isolation is provided among theneedle-like crystals 3. This configuration increases the effectivesurface area of the needle-like crystals 3.

In addition, because no insulator is used to isolate the transparentelectrode 2 side and the opposite side to the transparent electrode 2,neither an inconvenience resulting from deterioration or coloring of theinsulator, nor diffusion of impurities into the device ascribed to theuse of the insulator will occur in the photovoltaic conversion element(or light emitting element) 1.

FIG. 2 is a schematic cross section of a photovoltaic conversion elementaccording to a second embodiment of the invention. In FIG. 2, portionscorresponding to the respective portions shown in FIG. 1 are labeledwith the same reference characters as those of FIG. 1, and descriptionsthereof are omitted herein.

In the photovoltaic conversion element 1A, a composite 20 includes aplurality of third needle-like crystals (external needle-like crystals)3C extending from the tip ends of particular (part of) first needle-likecrystals 3A. The first needle-like crystals 3A without the thirdneedle-like crystals 3C extending from their tip ends are also present.Of the first needle-like crystals 3A, those having the third needle-likecrystals 3C extending from the tip ends are present, for example, one inevery 10000 to 50000 crystals. The third needle-like crystals 3C extendto the outside of the array region of the first needle-like crystals 3A.The third needle-like crystals 3C also extend in random directions(plural axial directions) within an angular range within which nointerference with the array 4 occurs.

The presence of the third needle-like crystals 3C increases the actualsurface area of the first through third needle-like crystals 3A through3C for the projected area onto the transparent electrode 2 in comparisonwith the case of the photovoltaic conversion element 1 shown in FIG. 1.Hence, in a case where the third needle-like crystals 3C are made of ann-type semiconductor (preferably, the same materials as the firstneedle-like crystals 3A), in comparison with the case of thephotoelectric element 1 shown in FIG. 1, it is possible to increase thearea of the p-n junction interface, which can in turn increase aphotoelectric current to be generated.

Also, in the photovoltaic conversion element 1A, the third needle-likecrystals 3C grown in plural axial directions can achieve an advantage ofscattering light. Hence, as is indicated by an arrow L in FIG. 2, lightcoming incident from the transparent 2 side and passing through thearray 4 once without being absorbed in the p-n junction interface isreflected by the third needle-like crystals 3C to be introduced to thep-n junction portion. The conversion efficiency of the photovoltaicconversion element 1A can be thus enhanced.

The third needle-like crystals 3C are not necessarily n-typesemiconductors. In this case, the advantage of increasing aphotoelectric current owing to an increase of the area of the p-njunction interface cannot be achieved. Nevertheless, it is possible toachieve an advantage of increasing the conversion efficiency byreflecting light having passed through the array 4 onto the p-n junctioninterface.

The structure of this photovoltaic conversion element 1A can be appliedto a light emitting element. In this case, because light emitted fromthe vicinity of the p-n junction interface to the opposite side to thetransparent electrode 2 can be introduced toward the transparentelectrode 2 side by reflecting (scattering) the light by the thirdneedle-like crystals 3C, it is possible to enhance the externalextraction efficiency for generated light.

FIG. 3 is a schematic cross section of a photovoltaic conversion elementaccording to a third embodiment of the invention. In FIG. 3, portionscorresponding to the respective portions shown in FIG. 1 are labeledwith the same reference characters as those of FIG. 1, and descriptionsthereof are omitted herein.

In the photovoltaic conversion element 11, the surface of theneedle-like crystals 3 is covered with a light receiving layer 9containing a dye in a space on the opposite side to the transparentelectrode 2 with respect to the array 4. By covering the surface of thetransparent electrode 2 with the needle-like crystals 3 almostcompletely, the contact area of the light receiving layer 9 and thetransparent electrode 2 is reduced. The light receiving layer 9 has athickness not to fill up a space between adjacent first needle-likecrystals 3A.

The surface of the light receiving layer 9 (the surface on the oppositeside to the surface of the needle-like crystals 3) is covered with thep-type semiconductor portion 5. A space between adjacent firstneedle-like crystals 3A is almost completely filled up with the lightreceiving layer 9 and the p-type semiconductor portion 5.

The photovoltaic conversion element 11 is able to generate aphotoelectric current using light absorbed in the light receiving layer9. A leak current can be reduced by reducing the contact area of thelight receiving layer 9 and the transparent electrode 2. In other words,in a case where the light receiving layer 9 is in contact with thetransparent electrode 2, holes are given to the transparent electrode 2from the light receiving layer 9 at a certain probability. Meanwhile,electrons are given to the transparent electrode 2 from the needle-likecrystals 3. This increases the recombination probability of electronsand holes in the transparent electrode 2 and the light emittingefficiency is reduced. It is, however, possible to avoid such an eventby reducing the contact area of the light receiving layer 9 and thetransparent electrode 2.

FIG. 4 is a schematic cross section showing the structure of a capacitoraccording to the first embodiment of the invention.

The capacitor 21 comprises a composite 30 including a storage node 22 inthe shape of a flat plate and serving as the base and an array 24 of aplurality of needle-like crystals 23 formed on the storage node 22. Bothof the storage node 22 and the needle-like crystals 23 have theconducting property and serve as one of capacitor electrodes of thecapacitor 21. The storage node 22 and the needle-like crystals 23 aremade of, for example, zinc oxide that is rendered electricallyconductive by aluminum doped therein.

The needle-like crystals 23 include first needle-like crystals 23Aextending from respective plural starting points S positioned spacedapart from one another on the surface of the storage node 22 at an angleof almost 90° with respect to the surface of the storage node 22(perpendicularly to the surface of the storage node 22), and secondneedle-like crystals 23B extending from the respective starting points Sin directions within a wider angular range encompassing a directionperpendicular to the surface of the storage node 22 (for example, theangle produced with the surface of the storage node 22 is 0° to 90°) andshorter than the first needle-like crystals 23A. In the first and secondneedle-like crystals 23A and 23B, the longitudinal directions and thec-axes coincide with each other. The first needle-like crystals 23A havealmost the same length.

One or more than one first needle-like crystal 23A and one or more thanone second needle-like crystal 23B extend from a single starting pointS. A clearance is present between adjacent two first needle-likecrystals 23A. The tip end (the end portion on the opposite side to thestorage node 22) of the second needle-like crystal 23B extending from agiven starting point S abuts on or comes in close proximity to the sidesurface of the first needle-like crystal 23A or the second needle-likecrystal 23B extending from another starting point S adjacent to thegiven starting point S.

A portion on the surface of the storage node 22 exposed through thefirst needle-like crystals 23A is substantially completely covered withthe second needle-like crystals 23B. In short, the surface of thestorage node 22 is covered with the needle-like crystals 23 (first andsecond needle-like crystals 23A and 23B) almost completely. In otherwords, the entire surface of the storage node 22 is covered with theneedle-like crystals 23. Hence, the storage node 22 side and theopposite side to the storage node 22 with respect to the array 24 areisolated in a satisfactory manner.

The second needle-like crystals 23B are present in a region(hereinafter, referred as the “first region”) R21 in the vicinity of thesurface of the storage node 22 with respect to a direction perpendicularto the surface of the storage node 22, and absent in a region(hereinafter, referred to as the “second region”) R22 on the tip endside of the first needle-like crystals 23A within an array region of theneedle-like crystals 23. Hence, a proportion of the cross section (area)of the needle-like crystals 23 in a plane parallel to the surface of thestorage node 22 is lower in the second region R22 than in the firstregion R21. The orientation degree of the needle-like crystals 23 is lowin the first region R21 in comparison with the second region R22. Theneedle-like crystals 23 as a whole (at least in the second region R22)are oriented with respect to the c-axes thereof.

A plurality of microscopic regions 27 made of crystals of zinc oxide arepresent between the storage node 22 and the array 24 (chiefly in thevicinity of the starting points S). The crystal orientations of therespective microscopic regions 27 are random and do not necessarilycoincide with the crystal orientations of the needle-like crystals 23.

The surface of the needle-like crystals 23 is covered with a protectivefilm (insulating film) 25 made of a dielectric material in a space onthe opposite side to the storage node 22 with respect to the array 24.By covering the surface of the storage node 22 with the needle-likecrystals 23 almost completely, the contact area of the protective film25 and the storage node 22 is reduced. The protective film 25 has athickness not to fill up a space between adjacent first needle-likecrystals 23A. The protective film 25 is made of, for example, non-dopedzinc oxide.

The surface of the protective film 25 (the surface on the opposite sideto the surface of the needle-like crystals 23) is covered with a cellplate 26. The cell plate 26 has the conducting property and serves asthe other capacitor electrode. The cell plate 26 is made of, forexample, aluminum-doped zinc oxide. A space between adjacent firstneedle-like crystals 23A is almost completely filled up with theprotective film 25 and the cell plate 26. The surface of the cell plate26 (the surface on the opposite side to the surface of the protectivefilm 25) is covered with another protective film (insulating film) 28.The protective film 28 is made of, for example, non-doped zinc oxide.

As has been described, the capacitor 21 has the structure in which theprotective film 25, which is a dielectric material, is sandwichedbetween the needle-like crystals 23 (and the storage node 22) serving asone of the capacitor electrodes and the cell plate 26 serving as theother capacitor electrode. When a voltage is applied between the storagenode 22 and the cell plate 26, the capacitor 21 becomes able to storecharges in the vicinity of the interface between the needle-likecrystals 23 and the protective film 25 and in the vicinity of theinterface between the cell plate 26 and the protective film 25. Becausethe protective film 25 is of a shape conforming to the surface of theneedle-like crystals 23, the area of the interfaces between theprotective film 25 and the respective needle-like crystals 23 and cellplate 26 is increased. The electrostatic capacity of the capacitor 21 istherefore large.

In addition, the protective film 25 is hardly in contact with thestorage node 22 whereas it is in contact with virtually the array 24 ofneedle-like crystals 23 alone on the opposite side to the cell plate 26with respect to the protective film 25. Hence, in a case where theneedle-like crystals 23 are made of a single kind of material (oxide),it is possible to apply a voltage across the protective film 25homogeneously. In addition, no electric double layer is formed betweenthe storage node 22 and the array 24. It is thus possible for thecapacitor 21 to enhance the electrostatic capacity.

Further, because the needle-like crystals 23 as a whole are orientedwith respect to the c-axes thereof, at least in the second region R22,the electric property thereof is homogenous with respect to the in-planedirection perpendicular to the orientation direction.

As has been described, the capacitor 21 can be formed of the storagenode 22 through the protective film 28, all of which are made of zincoxide with a difference only as to whether the zinc oxide used is dopedor not. It goes without saying, however, that the respective portionsincluding the storage node 22 through the protective film 28 (eitherpartially or entirely) can be made of a material other than zinc oxide.

FIG. 5 is a schematic cross section showing the structure of a capacitoraccording to the second embodiment of the invention. In FIG. 5, portionscorresponding to the respective portions shown in FIG. 4 are labeledwith the same reference characters as those of FIG. 4, and descriptionsthereof are omitted herein.

The capacitor 31 is a so-called electric double layer capacitor, andcomprises a composite 40 including a substrate (base) 32 serving as acollector and an array 34 of needle-like crystals 33 in the samemorphology as the needle-like crystals 23 (see FIG. 4) that form one ofpolarizable electrodes of the capacitor 31 together with the substrate32. The needle-like crystals 33 include first needle-like crystals 33Ain the same morphology as the first needle-like crystals 23A and secondneedle-like crystals 33B in the same morphology as the secondneedle-like crystals 23B. The surface of the substrate 32 is coveredwith the needle-like crystals 33 (first and second needle-like crystals33A and 33B) substantially completely.

Another substrate 36 functioning the other polarizable electrode toserve as a collector is disposed oppositely to the substrate 32 whilebeing spaced apart adequately from the tip ends of the first needle-likecrystals 33A. The substrate 36 is made of the same material as thesubstrate 32.

A space between the array 34 and the substrate 36 (including a spacebetween adjacent first needle-like crystals 33A and a space between thetip ends of the first needle-like crystals 33A and the substrate 36) isfilled with an electrolytic solution 35. By covering the surface of thesubstrate 32 with the needle-like crystals 33 substantially completely,the contact area of the electrolytic solution 35 and the substrate 32 isreduced.

When a voltage is applied between a pair of the substrates 32 and 36, anelectric double layer is formed in the vicinity of the interfacesbetween the respective substrates 32 and 36 and the electrolyticsolution 35, which enables the capacitor 31 to store charges.

The use of the needle-like crystals 33 as one of the polarizableelectrodes increases the area of the interface between the needle-likecrystals 33 and the electrolytic solution 35 at which the electricdouble layer is formed, which increases the electrostatic capacity ofthe capacitor 31. It is preferable that a space between adjacent firstneedle-like crystals 33A forms a mesopore in the order of 2 nm to 50 nmin width. This configuration makes it possible to increase an amount ofstored charges. In addition, because the contact area of theelectrolytic solution 35 and the substrate 32 is reduced, no leakcurrent resulting from a supporting salt in the electrolyte will occur.In short, a leak current is reduced in this capacitor 31.

Further, activated carbon in the form of an aggregate of fine particleshas been used as the polarizable electrode in the conventional electricdouble layer capacitor. However, the electric resistance is high becauseof an influence of the grain boundary or the like. On the contrary,because the needle-like crystals 33, that is, needle-like crystals whosemajor portion are single crystals, are used as the polarizable electrodein the capacitor 31, there is little influence of the grain boundary andthe polarizable electrode (needle-like crystals 33) has a highconducting property. The electric conductivity of the needle-likecrystals 33 can be controlled by controlling an amount of doping.

Needle-like crystals same as the needle-like crystals 33 may extend fromthe substrate 36 toward the substrate 32. In this case, it is possibleto dispose the needle-like crystals from the substrate 36 and theneedle-like crystals 33 from the substrate 32 oppositely so as not comeinto contact with each other. Accordingly, it is possible to increasethe area of the interface between the electrolytic solution 35 and thepolarizable electrode (the needle-like crystals extending from thesubstrate 36) on the substrate 36 side, too. The electrostatic capacityof the capacitor can be therefore increased.

A method for producing the composites 10, 20, 30, and 40 will now bedescribed. The composites 10, 20, 30 and 40 can be obtained by lettingthe needle-like crystals 3, 23, and 33 be grown on the transparentelectrode 2, the storage node 22, or the substrate 32 serving as thebase by means of electroless plating using a plating solution(hereinafter, referred to as the “specific plating solution”) containingat least one kind of alkali selected from the group consisting of X¹OH,where X¹ is one of Na, K, and Cs, X² ₂CO₃, where X² is one of H, Na, K,and Cs, and NH₃₁ and having a pH of 13 or higher.

In this case, the needle-like crystals 3, 23, and 33 are grown using asubstance having a hydroxyl group on the surface thereof as a catalyst.

The foundation for the growth of the needle-like crystals 3, 23, and 33satisfies either one of the requirement that it is made of a pluralityof crystal grains that are crystal grains having random crystalorientations and the requirement that it has a hydrophilic surface andis amorphous. An amorphous matrix may be present in a space between acrystal grain and another crystal grain. In a case where the surfaceportion itself of the base satisfies either one of these requirements,the needle-like crystals 3, 23, and 33 can be grown using the surfaceportion of the base as the foundation.

Alternatively, particles as the foundation such that satisfies eitherone of the requirements specified above, for example, particles made ofthe same material as the microscopic regions 7 and 27, may be placed onthe surface of the base before the electroless plating is performed. Inthis case, the needle-like crystals 3, 23, and 33 are grown using theparticles as the seeds in the step of performing the electrolessplating. Hence, in this case, it is possible to control the density ofthe arrays 4, 24, and 34, respectively, of the needle-like crystals 3,23, and 33 that will be grown with the density of particles to beplaced.

In a case where the electroless plating is performed without placingsuch particles on the surface of the base, the base itself has to be asubstance having a hydroxyl group on the surface thereof (it does notmatter whether it is a substance that contains an OH-group in thestructure formula or it is a substance that forms an OH-group on thesurface upon contact with water or the like). Meanwhile, in a case wherethe electroless plating is performed after the particles are placed onthe surface of the base, it is sufficient that the particles are asubstance that has a hydroxyl group on the surface thereof and the basedoes not have to be a substance having a hydroxyl group on the surfacethereof.

According to the producing method described above, it is possible toproduce the composites 10, 20, 30, and 40 in which the needle-likecrystals 3, 23, and 33 are formed at a high density that has not beenachieved before. The density of the needle-like crystals 3, 23, and 33in the second regions R2 and R22 (that is, the density of the firstneedle-like crystals 3A, 23A, and 33A) measured from a scanning electronmicrograph taken from the tip end side of the first needle-like crystals3A, 23A, and 33A is, for example, 1000 crystals or more per a projectedarea of 1 μm×1 μm.

By means of electroless plating, not only is it possible to form thearrays 4, 24, and 34 of the homogeneous needle-like crystals 3, 23, and33, respectively, even on the base having a complicated surface shape,but it is also possible to form the arrays 4, 24, and 34 when the basehas no conducting property. Accordingly, various bases become availablein comparison with electrolytic plating, which broadens the applicablerange of this producing method.

In addition, the forming temperature of the needle-like crystals 3, 23,and 33 by means of electroless plating is 60° C. to 100° C., andpreferably 70° C. to 90° C., which eliminates the need to hold the baseat high temperatures as in the case of chemical vapor deposition orpulse laser deposition. It is therefore possible to use a material thatcannot be used for a reaction at high temperatures, such as plastic, asthe base.

Further, according to this producing method, the needle-like crystalsare grown to be the needle-like crystals 3, 23, and 33 in variousdirections from the starting points on the surface of the base (in acase where the producing method includes the step of placing theparticles on the surface of the base, the particles become the startingpoints). This is a noticeable characteristic in comparison with theconventional method for producing needle-like crystals, such as chemicalvapor deposition (for example, see Non-Patent Document 1 supra) andelectrochemical deposition (for example, see Non-Patent Document 2supra) where most of the needle-like crystals are grown in a directionalmost perpendicular to the surface of the base.

When a plurality of starting points for growth of needle-like crystalsare positioned spaced apart from one another on the surface of the base,the surface of the base is exposed through the needle-like crystalsgrown almost perpendicularly to the surface of the base. On thecontrary, according to the producing method of the present invention, aportion on the surface of the base exposed through the needle-likecrystals (corresponding virtually to the first needle-like crystals 3A,23A, and 33A) grown almost perpendicularly to the surface of the base(in such a manner that the angle produced with the surface of the basefalls within a specific angular range) is covered with the needle-likecrystals (corresponding virtually to the second needle-like crystals 3B,23B, and 33B) grown in directions far from the direction perpendicularto the surface of the base substantially completely.

In other words, according to the producing method of the presentinvention, it is possible to produce the composites 10, 20, 30, and 40in which the base side and the opposite side to the base with respect tothe arrays 4, 24, and 34 can be isolated in a satisfactory manner. Thereis no need to use an insulator subsidiarily for this isolation.

Directions in which the needle-like crystals 3, 23, and 33 are grownalso depend on the crystal orientations or the crystal properties inportions that become the starting points from which the needle-likecrystals 3, 23, and 33 are grown. For example, in a case where the basehas a hydroxyl group, the base can be used as the starting points forgrowth. However, in a case where the crystal orientations of the baseare aligned, there may be a case where the crystal orientations of theneedle-like crystals 3, 23, and 33 are aligned during the growth. Forexample, in a case where an array of zinc oxide needle-like crystals isformed using a zinc oxide thin film formed by means of sputtering as theseeds (see Non-Patent Document 5 supra), the orientations of the zincoxide needle-like crystals are readily aligned because the crystalorientations have been aligned in the respective portions of the zincoxide thin film.

Hence, for portions that become the starting points from which theneedle-like crystals 3, 23, and 33 are grown, it is preferable thateither they show the crystal property and have random crystalorientations or they show no crystal property. For example, in a casewhere particles (a substance having a surface hydroxyl group) made ofthe same material as the microscopic regions 7 and 27 show the crystalproperty and have random crystal orientations or show no crystalproperty, it is possible to randomize the crystal orientations of theneedle-like crystals 3, 23, and 33 grown from the particles that becomethe seeds. It should be noted, however, that even when the particlesthat become the seeds have the crystal property, the crystal orientationof a given particle does not necessarily coincide with the crystalorientations of the needle-like crystals 3, 23, and 33 grown from thisparticle.

In this case, the needle-like crystals 3, 23, and 33 are grown in randomdirections and bond one to another because each becomes athree-dimensional hindrance to another, which consequently makes itpossible to cover the base substantially with the arrays 4, 24, and 34,respectively, of the needle-like crystals 3, 23, and 33 in the firstregions R1 and R21.

In a case where, for example, a colloidal solution in which aredispersed the particles (fine particles) that become the starting pointsfor growth of the needle-like crystals 3, 23, and 33 is applied to thebase by means of dip coating, spray coating, spin coating, or droppingwhen the particles are placed on the surface of the base, the crystalorientations of the particles will not align in most cases. In short, itis possible to place particles in random crystal orientations on thebase by these methods.

Also, according to the producing method of the present invention, theneedle-like crystals 3, 23, and 33 grown from arbitrary starting pointsin directions far from a direction perpendicular to the surface of thebase cannot be grown long as they are hindered by the needle-likecrystals 3, 23, and 33 grown from other adjacent starting points,whereas the needle-like crystals 3, 23, and 33 grown in a directionalmost perpendicular to the surface of the base can be grown longwithout being hindered by other needle-like crystals 3, 23, and 33. Inthis manner, the longitudinal directions of the needle-like crystals 3,23, and 33 as a whole are aligned. Hence, for example, in a case wherethe longitudinal directions of the needle-like crystals 3, 23, and 33coincide with the c-axes thereof, the arrays 4, 24, and 34 of theneedle-like crystals 3, 23, and 33, respectively, are oriented withrespect to the c-axes thereof.

In order to cover the base substantially with the needle-like crystals3, 23, and 33, it is necessary that the starting points for growth arepresent at some degree of density. Hereinafter, assume that the staringpoints for growth are particles (fine particles).

In a case where the arrays 4, 24, and 34 of the needle-like crystals 3,23, and 33, respectively, made of zinc oxide are formed, particles (fineparticles made of zinc oxide) that become the starting points for growthcan be formed, for example, by spin coating an ethanol solution, inwhich zinc acetate dihydrate (a precursor of zinc oxide) is dissolved,to the surface of the base and subjecting the base to heating(calcination) to let the zinc oxide undergo heat decomposition. It ispossible to obtain fine particles made of amorphous zinc oxide and/orfine particles made of crystalline zinc oxide having random crystalorientations depending on the heating temperature. In addition, theremay be generated a state where an amorphous matrix made of zinc oxide ispresent in spaces among fine particles made of crystalline zinc oxidehaving random crystal orientations.

In this case, by setting the concentration of zinc acetate dihydrate inthe ethanol solution to about 0.001 mol/l to about 0.1 mol/l, fineparticles made of zinc oxide can be present densely (for example,present so as to form a thin film) to the extent that the needle-likecrystals 3, 23, and 33 to be grown will substantially cover the base.

In a case where the concentration of zinc acetate dihydrate in theethanol solution is too low, the density of fine particles formed on thesurface of the substrate becomes too low for the needle-like crystals 3,23, and 33 to substantially cover the base. It should be noted, however,that even when the concentration is 0.001 mol/l or below, by increasingthe number of performing times of spin coating and heat decomposition,particles that become the staring points for growth of the needle-likecrystals 3, 23, and 33 can be present densely to the extent that theneedle-like crystals 3, 23, and 33 will cover the base densely.

In this case, however, fine particles flocculate partially and theneedle-like crystals 3, 23, and 33 are formed on the flocculated fineparticles, which makes it impossible to increase the adhesion strengthbetween the base and the needle-like crystals 3, 23, and 33 (arrays 4,24, and 34). Hence, there may be a case where the base is exposed due toseparation of the needle-like crystals 3, 23, and 33. It is thereforepreferable to perform the spin coating one to five times.

Meanwhile, when the concentration is higher than about 0.1 mol/l,needle-like crystals 3, 23, and 33 are formed on the flocculated fineparticles.

It should be noted that when the needle-like crystals 3, 23, and 33 madeof zinc oxide are grown, fine particles made of zinc oxide are takeninto the needle-like crystals 3, 23, and 33. Hence, even in a case wherethe needle-like crystals and the base do not have a direct contact witheach other at the beginning of the growth of the needle-like crystals 3,23, and 33, the needle-like crystals 3, 23, and 33 and the surface ofthe base can come into direct contact with each other by the time thegrowth of the needle-like crystals 3, 23, and 33 completes. At a pointin time when the growth of the needle-like crystals 3, 23, and 33completes, if an area where the needle-like crystals 3, 23, and 33 andthe base are in a direct contact is large to a certain extent, it ispossible to increase the adhesion strength between the base and theneedle-like crystals 3, 23, and 33.

As has been described, in a case where the needle-like crystals 3, 23,and 33 are formed on the flocculated fine particles, at a point timewhen the growth of the needle-like crystals 3, 23, and 33 completes,either the contact area of the base and the arrays 4, 24, and 34 becomessmaller or the base and the arrays 4, 24, and 34 are not in contact witheach other. The adhesion strength between the base and the needle-likecrystals 3, 23, and 33 consequently becomes weaker, which gives rise toseparation of the needle-like crystals 3, 23, and 33. The base istherefore exposed so that it is impossible to substantially cover thebase with the arrays 4, 24, and 34 of the needle-like crystals 3, 23,and 33, respectively.

Fine particles made of zinc oxide were obtained when the temperature ofheating following the spin coating was 180° C. to 500° C. Even in thistemperature range, in the case of heating at low temperatures, it isnecessary to extend the heating time to obtain fine particles made ofzinc oxide.

As long as the salt (precursor) used undergoes decomposition andparticles made of a target material can be obtained, hydrolysis,dehydrocondensation, or plasma irradiation may be performed instead ofheat decomposition (including decomposition by microwave heating).

The needle-like crystals 3, 23, and 33 may be grown repetitively byperforming electroless plating plural times. In this case, theneedle-like crystals 3, 23, and 33 made of the same substance(composition) do not have to be grown in all the times the electrolessplating is performed, and the needle-like crystals 3, 23, and 33 made ofdifferent substances (compositions) may be grown each time theelectroless plating is performed. For example, aluminum-dopedneedle-like crystals 3, 23, and 33 can be grown by performingelectroless plating plural times with the use of a plating solutioncontaining an aluminum supply source. In this case, the concentration ofthe aluminum supply source in the plating solution may be lowered forelectroless plating performed later, so that an amount of aluminum dopedinto the needle-like crystals 3, 23, and 33 is lower.

The arrays 4, 24, and 34 formed in this manner are suitable for use in aphotovoltaic conversion element. Because an amount of aluminum dopedinto the needle-like crystals 3 is smaller on the second region R2 side,the inverse saturation current density becomes low, and because anamount of aluminum doped into the needle-like crystals 3 becomes largeron the first region R1 side, thereby the resistance value becomes small.

In a case where the needle-like crystals 3, 23, and 33 are formedrepetitively through growth, electroless plating does not have to beperformed using the specific plating solution specified above in all thetimes. Once the base is substantially covered with the needle-likecrystals 3, 23, and 33 (arrays 4, 24, and 34), the needle-like crystals3, 23, and 33 may be grown thereafter by means of electroless platingusing a plating solution other than the specific plating solution (forexample, a plating solution excluding the specific alkali specifiedabove, such as NaOH).

For example, the needle-like crystals 3, 23, and 33 may be grown furtherusing the arrays 4, 24, and 34 in the composites 10, 20, 30 and 40 thatcan be obtained in the present invention as the seeds by means ofelectroless plating using a plating solution containing hexamethylenetetramine (a plating solution that does not satisfy the requirements ofthe specific plating solution), electrochemical deposition, vapor phasedeposition, and the like. When configured in this manner, it is possibleto form the composites 10, 20, 30, and 40 having a high aspect ratio inwhich the base is substantially covered with the first regions R1 andR21.

In a case where the producing method includes the step of placing theparticles on the surface of the base, a part of the particles may remainas the microscopic regions 7 and 27 after the growth of the needle-likecrystals 3, 23, and 33 completes. In this case, it is possible toenhance the adhesion between the needle-like crystals 3, 23, and 33 andthe base by the microscopic regions 7 and 27. It is possible to controlthe adhesion strength between the base and the arrays 4, 24, and 34 withthe density of the particles placed on the surface of the base prior tothe electroless plating. It is also possible to weaken the adhesionstrength between the base and the arrays 4, 24, and 34 with an intentionto let the arrays 4, 24, and 34 separate from the base after the arrays4, 24, and 34 are formed.

In the producing method of the present invention, it is possible tocontrol the length and the density of the needle-like crystals 3, 23,and 33, for example, by controlling a time over which electrolessplating is performed. In a case where the composites 10, 20, 30, and 40obtained in this manner are applied to a capacitor, the electrostaticcapacity can be readily controlled.

While the embodiments of the invention have been described, it should beappreciated that the invention can be implemented in other embodiments.For example, each of the first needle-like crystals 3A and 23A extend atan angle of almost 90° with respect to the surface of the base (thetransparent electrode 2, the storage node 22, and the substrate 32) inthe embodiments above. However, they may extend in a broader angularrange, for example, within which the angle produced with the base is 45°to 90°. A composite having such first needle-like crystals can beproduced by placing particles serving as the seeds on the base at alower density to let needle-like crystals be grown therefrom.

It should appreciated that various modifications are possible within thescope of the appended claims.

EXAMPLES

Hereinafter, the invention will be described more concretely in Examplesbelow. It should be appreciated, however, that the invention is notlimited to Examples below.

Example 1

The following will describe a case where a composite comprising a baseand an array of needle-like crystals of oxide formed by means ofelectroless plating on the surface of the base was produced.

A glass substrate was prepared and used as the base. A zinc nitrateaqueous solution (0.001 mol/l) was applied on one of the surfaces of thebase (hereinafter, referred to the “surface”), and the zinc nitrateaqueous solution was spin coated on the surface of the base by rotatingthe base at 1500 rotations/min. Further, the base was dried at roomtemperature followed by calcination at 260° C. The base on the surfaceof which were placed fine particles of zinc oxide (foundation for thegrowth of needle-like crystals) was thus obtained.

Subsequently, the base on which were placed the fine particles wasdipped in an electroless plating solution within a reaction container(pressure-resistant container) made of Teflon (registered trademark).The electroless plating solution used was made up of water as a solventand zinc nitrate hexahydrate and sodium hydroxide (NaOH) as a solute.The concentration of zinc nitrate hexahydrate in the plating solutionwas 0.06 mol/l and the concentration of sodium hydroxide in the platingsolution was 0.75 mol/l. The pH of the plating solution was 13.2. Whenthe plating solution was heated at 85° C. for one hour within thereaction container, an array of needle-like crystals made of zinc oxidewas formed on the surface of the base.

As is shown in FIG. 6, while the base 124 was dipped in the platingsolution 123 (including the time of heating), the base 124 was supportedusing a supporter 125 in a posture for the surface of the base 124 (thesurface on the side where needle-like crystals were to be grown) to facethe bottom surface of the reaction containers 121 and 122 (faceddownward). When configured in this manner, the density of needle-likecrystals grown in plural axial directions from the tip ends of theneedle-like crystals that have been grown directly from the base likethe third needle-like crystals 3C shown in FIG. 2 becomes lower, whichmakes it possible to obtain an array with an excellent lighttransmitting property.

In a case where the producing method using electrolytic plating insteadof electroless plating is adopted, the base is limited to a substancehaving the conducting property. It is therefore difficult to form anarray of needle-like crystals on the glass substrate as is in thisexample. In addition, in a case where an array of needle-like crystalsis formed on the base having a complicated shape, such as a designedarticle, it is difficult to control the electric field concentration inthe case of electrolytic plating. However, in the case of electrolessplating, a homogeneous array can be readily formed while the producingapparatus is left simple. Further, in a case where an array ofneedle-like crystals is formed on a large base, provided that only thesolvent, the catalyst, and the temperature are placed under the sameconditions in essentials, the growing rate of needle-like crystals isalmost the same in every point of the base in the case of electrolessplating. An array having a uniform thickness can be therefore readilyobtained.

FIG. 7A shows scanning electron microscope images of an array obtainedin this example (taken in a direction looking down the baseperpendicularly (top) and in a direction parallel to the base (bottom)).The diameter of the needle-like crystals was about 80 nm and the lengthof the needle-like crystals was about 1.4 μm. The density of theneedle-like crystals in this instance was about 1800 crystals per squareμm.

FIG. 7B is a scanning electron microscope image of the array separatedfrom the bas substance, which is taken from the side where the base wasformerly present. In comparison with the opposite side to the base (thesecond region, see FIG. 7A), the needle-like crystals were presentdensely on the base side (the first region, see FIG. 7B) in this array.

For the purpose of comparison, a base on the surface of which wereplaced fine particles of zinc oxide was dipped in a plating solutionunder the same conditions as in the example above except that theconcentration of sodium hydroxide in the plating solution was 0.005mol/l and the pH of the plating solution was 11.6. No needle-likecrystals were observed on the surface of the resulting base andparticles of zinc oxide adhered partly onto the base. The averageparticle diameter of the particles of zinc oxide was about 25 nm.

Also, for the purpose of comparison, a base on the surface of which wereplaced fine particles of zinc oxide was dipped in a plating solutionunder the same conditions as in the example above except that theplating solution used (hereinafter, referred to as the “plating solutionof Comparative Example”) contained zinc nitrate hexahydrate at aconcentration of 0.06 mol/l and hexamethylene tetramine at aconcentration of 0.75 mol/l. The pH of the plating solution ofComparative Example was found to be in the neighborhood of 7.0 whenconfirmed with a pH test paper.

When the surface of the resulting base was observed by a scanningelectron microscope from a direction perpendicular to the substrate, thebase was exposed through the array of needle-like crystals.

The length of the needle-like crystals after the base was dipped in theplating solution for one hour was 1.4 μm when the plating solution ofthe example (containing sodium hydroxide and having the pH of 13.2) wasused. In contrast, the length was in the order of 200 nm when theplating solution of Comparative Example was used. It is thereforeunderstood that the growing rate of the needle-like crystals is slow ina case where the plating solution of Comparative Example was used incomparison with a case where the plating solution of the example wasused.

Example 2

The following will describe a case where an array of needle-likecrystals made of zinc oxide was formed on the surface of the base bymeans of electroless plating and a dye sensitizing photovoltaic cell wasfabricated using the array as an n-type semiconductor.

A transparent conductive film (F-doped SnO₂ having a sheet resistance of12Ω/□) formed on one of the surfaces of a glass substrate was preparedand used as the base. The face of the glass substrate on which wasformed the transparent conductive film (the surface having theconducting property) was defined as the surface and a zinc nitrateaqueous solution mol/l) was applied on this surface. The zinc nitrateaqueous solution was spin coated on the glass substrate by rotating theglass substrate at 1500 rotations/min, after which the glass substratewas dried at room temperature followed by calcination at 260° C. Thebase on the surface of which were placed fine particles of zinc oxidewas thus obtained.

The base on which were placed the fine particles was dipped in anelectroless plating solution within a reaction container. Theelectroless plating solution used was made up of water as a solvent andzinc nitrate hexahydrate and sodium hydroxide as a solute atconcentrations in the aqueous solution of 0.06 mol/l and 0.75 mol/l,respectively. The pH of the plating solution was 13.5. Apressure-resistant container made of Teflon (registered trademark) wasused as the reaction container. As with Example 1, the base was dippedin the plating solution in a posture for the surface of the base to facethe bottom surface of the reaction container.

When the plating solution was heated at 85° C. for one hour within thereaction container, an array of needle-like crystals made of zinc oxidewas formed on the surface of the base. FIG. 8A shows a scanning electronmicroscope image (taken in a direction looking down the baseperpendicularly) of the array after the electroless plating was repeatedthree times. The diameter of the needle-like crystals was about 80 nmand the length of the needle-like crystals was about 5.2 μm. The densityof the needle-like crystals in this instance was about 1700 crystals persquare μm.

FIG. 8B is a view showing the X-ray diffraction pattern when the arraywas separated from the base and the array was measured on the side(first region) where the base was placed and on the opposite side to thesubstrate (second region).

In the first region, (002), (101), and (100) peaks were obtained.Meanwhile, in the second region, (002) and (101) peaks were obtained buta (100) peak was not obtained. Also, the (101) peak was extremely smallin the second region in comparison with that in the first region. Inshort, the orientation degree of the needle-like crystals in the firstregion was lower than the orientation degree of the needle-like crystalsin the second region.

Subsequently, an ethanol solution in which was dissolved a Ru complexwas held at 40 C.° and the array was dipped in this solution for 15minutes for the Ru dye to be absorbed onto the surface of the array. Thecounter electrode of the transparent conductive film (the base of thearray) used herein was prepared by vapor depositing 1000-angstrom-thickplatinum on a transparent conductive film (F-doped SnO₂ having a sheetresistance of 12Ω/□) formed on one of the surfaces of a glass substrate.A spacer was interposed between the counter electrode and the base sothat a minute space was defined between the counter electrode and thebase, and an electrolytic solution was injected into this minute space.A solution prepared by dissolving lithium iodide and iodine inacetonitrile was used as the electrolytic solution. A dye sensitizingphotovoltaic cell was thus obtained.

In comparison with a dye sensitizing photovoltaic cell using fineparticles of zinc oxide instead of the needle-like crystals made of zincoxide, an amount of current was increased by 25% in this dye sensitizingphotovoltaic cell when a comparison was made with the one having thesame amount of dye.

Example 3

The following will describe a case where an array including an aggregateof needle-like crystals made of zinc oxide and grown in plural axialdirections was formed by means of electroless plating and a dyesensitizing photovoltaic cell was fabricated using the array as ann-type semiconductor.

A transparent conductive film (F-doped SnO₂ having a sheet resistance of12Ω/□) formed on one of the surfaces of a glass substrate was preparedand used as the base. The face of the glass substrate on which wasformed the transparent conductive film (the surface having theconductive property) was defined as the surface and a zinc nitrateaqueous solution (0.001 mol/l) was applied on this surface. The zincnitrate aqueous solution was spin coated on the glass substrate byrotating the glass substrate at 1500 rotations/min, after which theglass substrate was dried at room temperature followed by calcination at260° C. The base on the surface of which were placed fine particles ofzinc oxide was thus obtained.

The base on which were placed the fine particles was dipped in anelectroless plating solution within a reaction container. Theelectroless plating solution used was made up of water as a solvent andzinc nitrate hexahydrate and sodium hydroxide as a solute atconcentrations in the aqueous solution of 0.04 mol/l and 0.75 mol/l,respectively. The pH of the plating solution was 13.4. Apressure-resistant container made of Teflon (registered trademark) wasused as the reaction container. As with Example 1, the base was dippedin the plating solution in a posture for the surface of the base to facethe bottom surface of the reaction container.

When the plating solution was heated at 85° C. for one hour within thereaction container, an array of needle-like crystals made of zinc oxidewas formed on the surface of the base. FIG. 9 shows a scanning electronmicroscope image of the array (first sample) after the electrolessplating was repeated three times. The diameter of the needle-likecrystals was about 65 nm in average and the length of the needle-likecrystals was about 2.1 μm. The density of the needle-like crystals inthis instance was about 1800 crystals per square μm. On the array,aggregates of needle-like crystals (external needle-like crystals) madeof zinc oxide and grown in plural axial directions were presentsparsely. The density of the aggregates was about 0.03 count per 100squares μm.

Subsequently, an array (second sample) in which were formed aggregatesof external needle-like crystals at a higher density was prepared.Initially, a transparent conductive film (F-doped SnO₂ having a sheetresistance of 12Ω/□) formed on one of the surfaces of a glass substratewas prepared and used as the base. The face of the glass substrate onwhich was formed the transparent conductive film (the surface having theconducting property) was defined as the surface and a zinc nitrateaqueous solution (0.001 mol/l) was applied on this surface. The zincnitrate aqueous solution was spin coated on the glass substrate byrotating the glass substrate at 1500 rotations/min, after which theglass substrate was dried at room temperature followed by calcination at260° C. The base on the surface of which were placed fine particles ofzinc oxide was thus obtained.

The base on which were placed the fine particles was dipped in anelectroless plating solution within a reaction container. Theelectroless plating solution used was made up of water as a solvent andzinc nitrate hexahydrate and sodium hydroxide as a solute atconcentrations in the aqueous solution of 0.04 mol/l and 0.75 mol/lrespectively. The pH of the plating solution was 13.5. Apressure-resistant container made of Teflon (registered trademark) wasused as the reaction container. The base was dipped in the platingsolution in a posture for the surface of the base to face the oppositeside to the bottom surface of the reaction container. When configured inthis manner, the density of aggregates of needle-like crystals grown inplural axial directions becomes higher, which makes it possible toobtain an array having an excellent light scattering property.

When the plating solution was heated at 85° C. for one hour within thecontainer, an array of needle-like crystals made of zinc oxide wasformed on the surface of the base. FIG. 10 shows a scanning electronmicroscope image of the array after the electroless plating was repeatedthree times. The diameter of the needle-like crystals was in theneighborhood of 65 nm in average and the length of the needle-likecrystals was about 3.5 μm. Aggregates of needle-like crystals (externalneedle-like crystals) grown in plural axial directions were formed onthe array (needle-like crystals grown directly on the substrate) at adensity higher than in the first sample (almost across the entiresurface of the array).

The density of the needle-like crystals grown directly on the base inthis instance was about 1800 crystals per square μm. The density ofaggregates of external needle-like crystals was about 9 counts per 100squares μm.

Subsequently, a dye was absorbed into these two arrays (first and secondsamples). Initially, an ethanol solution in which was dissolved a Rucomplex was held at 40 C.° and the arrays were dipped in this solutionfor 15 minutes for the Ru dye to be absorbed onto the surface of eacharray. The counter electrode of the transparent conductive film (thebase of the array) used herein was prepared by vapor depositing1000-angstrom-thick platinum on a transparent conductive film (F-dopedSnO₂ having a sheet resistance of 12Ω/□) formed on one of the surfacesof a glass substrate. A spacer was interposed between the counterelectrode and the base so that a minute space was defined between thecounter electrode and the base, and an electrolytic solution wasinjected into this minute space. A solution prepared by dissolvinglithium iodide and iodine in acetonitrile was used as the electrolyticsolution. Dye sensitizing photovoltaic cells respectively using thefirst and second samples were thus obtained.

The dye sensitizing photovoltaic cell using the second sample had anamount of current per unit dye amount 30% higher than that in the dyesensitizing photovoltaic cell using the first sample. Also, an amount ofabsorbed dye in the second sample was twice as large as that in thefirst sample, and the conversion efficiency of the dye sensitizingphotovoltaic cell using the second sample was increased by a factor oftwo.

Example 4

The following will describe a case where an organic-inorganic hybridsolid state photovoltaic cell was fabricated. In this example, an arrayof needle-like crystals made of zinc oxide as an n-type semiconductorwas formed on the surface of the base by means of electroless plating,and poly-3-hexylthiophene was used as a p-type charge transporting layerand a light receiving material.

A transparent conductive film (F-doped SnO₂ having a sheet resistance of12Ω/□) formed on one of the surfaces of a glass substrate was preparedand used as the base. The face of the glass substrate on which wasformed the transparent conductive film (the surface having theconducting property) was defined as the surface and a zinc nitrateaqueous solution (0.001 mol/l) was applied on this surface. The zincnitrate aqueous solution was spin coated on the glass substrate byrotating the glass substrate at 1500 rotations/min, after which theglass substrate was dried at room temperature followed by calcination at260° C. The base on the surface of which were placed fine particles ofzinc oxide was thus obtained.

The base on which were placed the fine particles was dipped in anelectroless plating solution within a reaction container. Theelectroless plating solution used was made up of water as a solvent andzinc nitrate hexahydrate and sodium hydroxide as a solute atconcentrations in the aqueous solution of 0.06 mol/l and 0.75 mol/l,respectively. The pH of the plating solution was 13.4. Apressure-resistant container made of Teflon (registered trademark) wasused as the reaction container.

When the plating solution was heated at 85° C. for one hour within thecontainer, an array of needle-like crystals made of zinc oxide wasformed on the surface of the base. When confirmed by a scanning electronmicroscope, the diameter of the needle-like crystals was found to beabout 65 nm in average and the length of the needle-like crystals wasfound to be about 1.3 μm. The density of the needle-like crystals inthis instance was about 1700 crystals per square μm.

The base was dipped in the plating solution in a posture for the surfaceof the base to face the bottom surface of the reaction container.

The counter electrode of the transparent conductive film (the base ofthe array) used herein was prepared by vapor depositing1000-angstrom-thick gold on a transparent conductive film (F-doped SnO₂having a sheet resistance of 12Ω/□) formed on a glass substrate. Thearray of needle-like crystals made of zinc oxide and the counterelectrode were opposed to each other and a spacer was interposed in aspace therebetween so that a minute space was defined between the arrayand the counter electrode. By repetitively performing a step ofinjecting a solution prepared by dissolving poly-3-hexylthiophene inchlorobenzene into this minute space followed by drying at 80° C.,poly-3-hexylthiophene was provided in this minute space.

In a case where poly-3-hexylthiophene including the array of needle-likecrystals made of zinc oxide obtained in the manner described above wasused, the photovoltaic conversion was enhanced by 50% in comparison witha case where fine particles of zinc oxide were used instead of thearray.

Example 5

The following will describe a case where plastic was used as the baseand an array of needle-like crystals was formed on the surface of thebase by means of electroless plating.

Polyvinyl alcohol processed to have a shape of a flat plate was preparedand used as the base. Subsequently, fine particles of zinc oxide wereprepared as follows. Initially, a first solution was obtained by addingzinc acetate dihydrate to 40 ml of methanol at a concentration of 0.01mol/l, and dissolving the former in the latter by heating at 60° C.Also, a second solution was obtained by adding a potassium hydroxide to20 ml of methanol at a concentration of 0.03 mol/l. The second solutionwas heated to 60° C. and dropped slowly into the first solution withstirring. After the resulting mixed solution was held at 60° C. foreight hours, fine particles of zinc oxide having an average particlediameter of about 10 nm were obtained in the mixed solution.

After the mixed solution was cooled, methanol was added to the mixedsolution and the mixed solution was mixed satisfactorily with stirring.Thereafter, the mixed solution was subjected to centrifugal separationat a rotation speed of about 3000 rotations/min to let the fineparticles of zinc oxide precipitate, and a supernatant was disposed of.The fine particles of zinc oxide were rinsed by repeating the processfrom the addition of methanol to the disposal of supernatant as abovefive times.

The resulting fine particles of zinc oxide were dispersed again inmethanol, which was spin coated on the base by rotating the base at 1500rotations/min followed by drying at room temperature. The fine particlesof zinc oxide were thus placed on the surface of the base. The base onwhich were placed the fine particles of zinc oxide was dipped in anelectroless plating solution within a reaction container. Theelectroless plating solution used was made up of water as a solvent andzinc nitrate hexahydrate and sodium hydroxide as a solute atconcentrations in the aqueous solution of 0.06 mol/l and 0.25 mol/l,respectively. The pH of the plating solution was 13.1. Apressure-resistant container made of Teflon (registered trademark) wasused as the reaction container. The base was dipped in the platingsolution in a posture for the surface of the base to face the bottomsurface of the reaction container.

When the plating solution was heated at 80° C. for 30 minutes within thereaction container, an array of needle-like crystals made of zinc oxidewas formed on the surface of the base. When confirmed by a scanningelectron microscope, the diameter of the needle-like crystals was foundto be in the neighborhood of 80 nm and the length of the needle-likecrystals was found to be about 0.6 μm. The density of the needle-likecrystals in this instance was about 1500 crystals per square μm.

In Examples 1 through 5 above, the concentration of zinc nitratehexahydrate used as a raw material of the zinc oxide needle-likecrystals was 0.04 mol/l or 0.06 mol/l. However, an array of needle-likecrystals can be obtained when the concentration of zinc nitratehexahydrate is 0.01 mol/l to 2 mol/l and more preferably, needle-likecrystals can be grown at a concentration of 0.03 mol/l to 0.07 mol/l.Also, in Examples 1 through 5 above, the concentration of sodiumhydroxide was 0.75 mol/l or 0.25 mol/l. However, an array of needle-likecrystals was obtained when the concentration of sodium hydroxide was 0.1mol/l to 1 mol/l.

Example 6

The following will describe a case where a comparison was made betweendye sensitizing photovoltaic cells respectively using, as the n-typesemiconductor, an array of zinc oxide needle-like crystals thatsubstantially covered the surface of the base and an array of zinc oxideneedle-like crystals formed for the base to be exposed prepared byelectroless plating.

A glass substrate on one of the surfaces of which was formed atransparent conductive film (F-doped SnO₂ having a sheet resistance of12Ω/□) was prepared. The face of the glass substrate on which was formedthe transparent conductive film (the surface having the conductingproperty) was defined as the surface and a zinc acetate ethanol solution(0.03 mol/l) was applied on this surface. The zinc acetate ethanolsolution was spin coated on the glass substrate by rotating the glasssubstrate at 1500 rotations/min, after which the glass substrate wasdried at room temperature followed by calcination at 260° C. The base onthe surface of which was formed a thin film made of fine particles ofzinc oxide was thus obtained. An average thickness of the resulting thinfilm (measured by an atom force microscope) was 7 nm.

The base on which were placed the fine particles (thin film) was dippedin an electroless plating solution within a reaction container forelectroless plating to take place. The electroless plating solution usedwas made up of water as a solvent and zinc nitrate hexahydrate andsodium hydroxide as a solute. The concentration of zinc nitratehexahydrate in the plating solution was 0.06 mol/l and the concentrationof sodium hydroxide in the plating solution was 0.75 mol/l. Apressure-resistant container made of polypropylene was used as thereaction container. As with Example 1, the base was dipped in theplating solution in a posture for the surface of the base to face thebottom surface of the reaction container.

When the plating solution was heated at 80° C. for one hour within thereaction container, an array of needle-like crystals made of zinc oxidewas formed on the surface of the base. The electroless plating wasrepeated twice.

The diameter of the resulting needle-like crystals was about 65 nm andthe length of the needle-like crystals was about 3.2 μm. The density ofthe needle-like crystals in this instance was about 1000 crystals persquare μm. In this manner, an array of zinc oxide needle-like crystalsthat covered the base densely (substantially) was obtained.

For the purpose of comparison, an array of needle-like crystals formedfor a part of the base to be exposed was produced. A glass substrate onone of the surfaces of which was formed a transparent conductive film(F-doped SnO₂ having a sheet resistance of 12Ω/□) was prepared. The faceof the glass substrate on which was formed the transparent conductivefilm (the surface having the conducting property) was defined as thesurface. After an isopropanol solution in which were dispersed fineparticles of zinc oxide was dip coated on this surface, the glasssubstrate was dried at room temperature followed by heating at 150° C. Athin film made of fine particles of zinc oxide was formed on the surfaceof the base by repeating these dip coating, drying, and calcining stepsthree times. An average thickness of the resulting thin film was 70 nm.

The base on which were placed the fine particles (thin film) was dippedin an electroless plating solution within a reaction container forelectroless plating to take place. The electroless plating solution usedwas made up of water as a solvent and zinc nitrate and sodium hydroxideas a solute. The concentration of zinc nitrate in the plating solutionwas 0.06 mol/l and the concentration of sodium hydroxide in the platingsolution was 0.75 mol/l. A pressure-resistant container made ofpolypropylene was used as the reaction container. As with Example 1, thebase was dipped in the plating solution in a posture for the surface ofthe base to face the bottom surface of the reaction container.

When the plating solution was heated at 80° C. for one hour within thereaction container, an array of needle-like crystals made of zinc oxidewas formed on the surface of the base. When the electroless plating wasrepeated twice, the diameter of the resulting needle-like crystals wasabout 80 nm and the length of the needle-like crystals was about 3.0 μm.The density of the needle-like crystals in this instance was about 900crystals per square μm. In this manner, the array of zinc oxideneedle-like crystals covering the base densely was obtained.

Before the following step (step of letting a Ru dye be absorbed) wasperformed, a part of the array separated from the base, which generateda portion where the base was exposed through the array.

Subsequently, an ethanol solution in which was dissolved an Ru complexwas held at 40° C., and the bases on which were respectively formed twokinds of arrays were dipped in the solution for 15 minutes for an Ru dyeto be absorbed in the surface of each array. An amount of absorbed Rudye was made equal in each array.

Subsequently, as the counter electrode of the transparent conductivefilm (the base of the array), an electrode was prepared by vapordepositing 1000-angstrom-thick platinum on a transparent conductive film(F-doped SnO₂ having a sheet resistance of 12Ω/□) formed on one of thesurfaces of a glass substrate. By interposing a spacer between thecounter electrode and the base on which the array (the Ru dye wasabsorbed in the surface thereof) was formed, a minute space was definedbetween the counter electrode and the array, and an electrolyticsolution was injected into this minute space. A solution prepared bydissolving lithium iodide and iodine in acetonitrile was used as theelectrolytic solution. Dye sensitizing photovoltaic cells were thusobtained.

FIG. 11 shows current (current density) versus voltage characteristicsof the resulting dye sensitizing photovoltaic cells measured underirradiation of artificial solar light. The curve b1 shows thecharacteristic of the dye sensitizing photovoltaic cell using the arrayof zinc oxide needle-like crystals through which a part of the base wasexposed, and the curve b2 shows the characteristic of the dyesensitizing photovoltaic cell using the array of zinc oxide needle-likecrystals substantially covering the base.

The conversion efficiency of the dye sensitizing photovoltaic cell usingthe array of zinc oxide needle-like crystals substantially covering thebase was about 2.5 times higher than that of the dye sensitizingphotovoltaic cell using zinc oxide needle-like crystals through which apart of the base was exposed.

Example 7

A composite (hereinafter referred to as the “base covered typecomposite”) in which the base was covered with an array of needle-likecrystals substantially completely and a composite (hereinafter, referredto as the “base exposed type composite”) in which the base was exposedthrough an array of needle-like crystals were produced. A current versusvoltage characteristic of the array was measured for each composite.

The producing conditions of the base covered type composite were asfollows.

A transparent conductive film (F-doped SnO₂ having a sheet resistance of12Ω/□) formed on one of the surfaces of a glass substrate was preparedand used as the base. The face of the glass substrate on which wasformed the transparent conductive film (the surface having theconducting property) was defined as the surface and a zinc acetateethanol solution (0.01 mol/l) was applied on this surface. The zincacetate ethanol solution was spin coated on the glass substrate byrotating the glass substrate at 1500 rotations/min, after which theglass substrate was dried at room temperature followed by calcination at260° C. The base on the surface of which were placed fine particles ofzinc oxide was thus obtained.

Subsequently, the base on which were placed the fine particles wasdipped in an electroless plating solution within a reaction container(pressure-resistant container) made of Teflon (registered trademark).The electroless plating solution used was made up of water as a solventand zinc nitrate hexahydrate and sodium hydroxide as a solute. Theconcentration of zinc nitrate hexahydrate in the plating solution was0.04 mol/l and the concentration of sodium hydroxide in the platingsolution was 0.75 mol/l. The pH of the plating solution was 13.6. Whenthe plating solution was heated at 80° C. for one hour within thereaction container, an array of needle-like crystals made of zinc oxideand substantially covering the surface of the base was formed on thesurface of the base.

The producing conditions for the base exposed type composite were asfollows.

A transparent conductive film (F-doped SnO₂ having a sheet resistance of12Ω/□) formed on one of the surfaces of a glass substrate was preparedand used as the base. The face of the glass substrate on which wasformed the transparent conductive film (the surface having theconducting property) was defined as the surface and a zinc acetateethanol solution (0.01 mol/l) was applied on this surface. The zincacetate ethanol solution was spin coated on the glass substrate byrotating the glass substrate at 1500 rotations/min, after which theglass substrate was dried at room temperature followed by calcination at260° C. The base on the surface of which were placed fine particles ofzinc oxide was thus obtained.

Subsequently, the base on which were placed the fine particles wasdipped in an electroless plating solution within a reaction container(pressure-resistant container) made of Teflon (registered trademark).The electroless plating solution used was made up of water as a solventand zinc nitrate hexahydrate and hexamethylene tetramine as a solute.The concentration of zinc nitrate hexahydrate in the plating solutionwas 0.04 mol/l and the concentration of hexamethylene tetramine in theplating solution was 0.06 mol/l. The pH of the plating solution was 8.When the plating solution was heated at 80° C. for one hour within thereaction container, an array of needle-like crystals made of zinc oxidewas formed on the surface of the base. The surface of the base wasexposed through the array of needle-like crystals.

In both the base covered type composite and the base exposed typecomposite, the needle-like crystals (array) were semiconductors. Whetherthe array covered the base was confirmed from cross sectional SEMimages.

FIG. 12 shows current versus voltage characteristics of the base exposedtype composite and the base covered type composite. To enable themeasurement, a gold electrode was formed by vapor depositing gold so asto cover the array on the opposite side to the base (on the secondregion side). The current versus voltage characteristics shown in FIG.12 were obtained by applying a voltage between the gold electrode andthe transparent electrode serving as the base in a dark place and bymeasuring a current flowing between these electrodes.

The base exposed type composite showed an ohmic characteristic asindicated by the curve a1, which reveals that the two electrodesshort-circuited. Meanwhile, the base covered type composite shows anon-linear (non-ohmic) current versus voltage characteristic asindicated by the curve a2, which reveals that the two electrodes did notshort-circuit.

Example 8

Arrays were formed using three kinds of seeds for the growth ofneedle-like crystals. In each case, needle-like crystals were formed onthe surface of the base by means of electroless plating. In thisexample, a transparent conductive film (F-doped SnO₂ having a sheetresistance of 12Ω/□) formed on one of the surfaces of a glass substratewas prepared and used as the base.

The face of the glass substrate on which was formed the transparentconductive film (the surface having the conducting property) was definedas the surface and a 2-methoxyethanol solution was spin coated on thissurface, after which the glass substrate was dried at 60° C. for 24hours followed by calcination at 500° C. in air for one hour. A thinfilm (a seed of the first kind) made of fine particles of zinc oxide andhaving an average film thickness of 90 nm was thus formed on the surfaceof the base. The 2-methoxyethanol solution used was prepared bydissolving zinc acetate dihydrate at a concentration of 0.01 mol/l andmonoethanolamine at a concentration of 0.01 mol/l in a solvent.

Also, the face of the glass substrate on which was formed thetransparent conductive film (the surface having the conducting property)was defined as the surface, and a zinc oxide thin film (a seed of thesecond kind) was formed on this surface by means of sputtering usingzinc oxide at a purity of 99.9% as the target. The average filmthickness of the thin film of zinc oxide was 50 nm.

Further, the face of the glass substrate on which was formed thetransparent conductive film (the surface having the conducting property)was defined as the surface, and a zinc acetate ethanol solution (0.03mol/l) was spin coated on this surface while the base was rotated at aspeed of 1500 rotations/min, after which the glass substrate was driedat room temperature followed by calcination at 260° C. for two hours. Abase on the surface of which was formed a thin film (a seed of the thirdkind) made of fine particles of zinc oxide was thus obtained. Theaverage film thickness of the thin film obtained in this manner wasfound to be about 8 nm through evaluation by an atomic force microscope.

Subsequently, the bases on which were respectively formed the seeds ofthree kinds were dipped in an electroless plating solution within areaction container for electroless plating to take place. Theelectroless plating solution used was made up of water as a solvent andzinc nitrate hexahydrate and sodium hydroxide as a solute. Theconcentration of zinc nitrate hexahydrate in the plating solution was0.06 mol/l and the concentration of sodium hydroxide in the platingsolution was 0.75 mol/l. The pH of the plating solution was 13.2. Apressure-resistant container made of polypropylene was used as thereaction container.

As with Example 1 above, each base was dipped in the plating solution ina posture for the surface of the base to face the bottom surface of thereaction container. After the plating solution was heated at 80° C. forone hour, the plating solution was replaced with a new plating solution.Then, the new plating solution was heated at 80° C. for one hour again.An array of needle-like crystals was thus obtained.

The average thickness of the array of needle-like crystals formed usingthe seed of the first kind was 3.6 μm. When the array was visuallyobserved immediately after the growth of crystals completed (beforerinsing), the base was substantially covered with the array. However,when the array (composite) was handled later, a part of the arrayseparated from the base. Cracking and separation were confirmed in partof the array when confirmed with the use of a scanning electronmicroscope, which reveals that it was not possible to cover the basewith the array.

The average thickness of the array of needle-like crystals formed usingthe seed of the second kind was 3.3 μm. When this array was confirmedwith the use of a scanning electron microscope, the first needle-likecrystals extending in a direction producing an angle with the surface ofthe base such that falls within a specific angular range and the secondneedle-like crystals extending in directions within a wider angularrange encompassing the specific angular range and shorter than the firstneedle-like crystals were confirmed. A range of angles produced betweenthe base and the second needle-like crystals was narrow and the base wasnot covered with the array.

The average thickness of the array of needle-like crystals formed usingthe seed of the third kind was about 3.4 μm. When this array wasconfirmed with the use of a scanning electron microscope, the arraycovered the base across the entire surface of the base where plating wasapplied and no separation of the array occurred.

Example 9

A composite of needle-like crystals made of zinc oxide was formed on abase under the same conditions as those in Example 1.

The resulting array of zinc oxide needle-like crystals had the samestructure as the array 4 shown in FIG. 2, and a plurality of thirdneedle-like crystals (external needle-like crystals) extended from thetip ends of a part of the first needle-like crystals.

FIG. 13A and FIG. 13B are views showing EBSP's when the array of zincoxide needle-like crystals was separated from the base and observed fromthe first region side and from the second region side, respectively. InFIG. 13A and FIG. 13B, a plane producing a larger angle with the 001plane is illustrated in a pale color (close to white).

It is understood from the EBSP (FIG. 13A) observed from the first regionside that the orientations of zinc oxide needle-like crystals vary inthe vicinity of the surface on the first region side of the array ofzinc oxide needle-like crystals.

In contrast, because the plane along the longitudinal direction of thezinc oxide needle-like crystals is perpendicular to the 001 plane, it isunderstood from the EBSP (FIG. 13B) observed from the second region sidethat the zinc oxide needle-like crystals as a whole are oriented withrespect to the c-axes thereof in the vicinity of the plane on the secondregion side of the array of zinc oxide needle-like crystals.

Referring to FIG. 13B, a region (denoted by alpha-numeral c1 in FIG.13B) where the orientations of needle-like crystal were disturbed (theorientation degree is different from those in the other regions) ispresent in part. This region with disturbed orientations corresponds toa region where external needle-like crystals (see the third needle-likecrystals 3C of FIG. 2) were present. In the EBSP of FIG. 13B, the arrayof needle-like crystals is oriented in the 001 direction except for thisregion (the region where zinc oxide needle-like crystals were orientedwith respect to the c-axes thereof is denoted as alpha-numeral c2 inFIG. 13B).

As has been described, by confirming the EBSP of the array ofneedle-like crystals, it is possible to confirm a difference of theorientation degree between the first region and the second region of thearray of needle-like crystals by avoiding an influence of the externalneedle-like crystals.

FIG. 14A and FIG. 14B are frequency distributions of the planeorientation of zinc oxide needle-like crystals obtained by analyzingEBSP's shown in FIGS. 13A and 13B, respectively. The abscissa is usedfor the angle produced between a plane in the observed region with the001 plane, and the ordinate is used for the frequency (area reference)of the plane with which the angle is produced.

On the first region side (see FIG. 14A), planes in all the planeorientations with the angle of gradient from the 001 plane ranging from0 degree to 90 degrees were confirmed. In contrast, on the second regionside (see FIG. 14B), the frequency was concentrated within an angularrange close to the 001 plane. A region with the angle of gradient closeto 90 degrees was also observed. This region, however, is a regioncorresponding to the external needle-like crystals.

Example 10

The following will describe a case where an array of needle-likecrystals made of zinc oxide was formed on the surface of the base bymeans of electroless plating and a dye sensitizing photovoltaic cell wasfabricated using the array as an n-type semiconductor.

A transparent conductive film (F-doped SnO₂ having a sheet resistance of12Ω/□) formed on one of the surfaces of a glass substrate was preparedand used as the base. The face of the glass substrate on which wasformed the transparent conductive film (the surface having theconducting property) was defined as the surface and a zinc acetateethanol solution (0.03 mol/l) was applied on this surface. The zincacetate aqueous solution was spin coated on the glass substrate byrotating the glass substrate at 1500 rotations/min, after which theglass substrate was dried at room temperature followed by calcination at260° C. The base on the surface of which were placed fine particles ofzinc oxide was thus obtained.

The base on which were placed the fine particles was dipped in anelectroless plating solution within a reaction container for electrolessplating to take place. The electroless plating solution used was made upof water as a solvent and zinc nitrate hexahydrate and sodium hydroxideas a solute at concentrations in the aqueous solution of 0.04 mol/l and0.75 mol/l, respectively. A pressure-resistant container made ofpolypropylene was used as the reaction container. As with Example 1, thebase was dipped in the plating solution in a posture for the surface ofthe base to face the bottom surface of the reaction container.

When the plating solution was heated at about 85° C. for one hour withinthe reaction container, an array of needle-like crystals made of zincoxide was formed on the surface of the base. In short, the needle-likecrystals were oriented. The diameter of the needle-like crystals wasabout 45 nm and the length of the needle-like crystals was about 1.8 μm.The density of the needle-like crystals in this instance was about 1000crystals per square μm.

For the purpose of comparison, a thin film (first thin film) containingrandomly oriented needle-like crystals was produced. After a reactionsolution, particles of zinc oxide as the seeds of growth, a stirrerwhose surface was covered with Teflon (registered trademark) were putinto a pressure-resistant container made of polypropylene, the lid wasclosed and the mixture was heated at 85° C. for one hour with magneticstirring at a rotation speed of 800 rotations/min. The needle-likecrystals were thus obtained.

The reaction solution used was made up of water as a solvent and zincnitrate hexahydrate and sodium hydroxide as a solute at concentrationsin the aqueous solution of 0.04 mol/l and 0.75 mol/l, respectively. Anamount of the reaction solution used was 40 ml. The average particlediameter of the particles of zinc oxide was 20 nm and an amount of addedparticles of zinc oxide was 0.01 g.

According to this method, a reaction solution in which were dispersedzinc oxide needle-like crystals was obtained. The reaction solution wassubjected to centrifugal separation at a rotation speed of 10000rotations/min for ten minutes to isolate the supernatant from theprecipitate, and the supernatant was disposed of while distilled waterwas added to the precipitate. This operation was performed repetitivelyuntil the pH of the supernatant became about 7. The resultingprecipitate was dried at 40° C. and wet-kneaded with acetylacetone,polyethylene glycol, and distilled water. A paste containing zinc oxideneedle-like crystals was thus obtained.

An operation to apply this paste to a transparent conductive film(F-doped SnO₂ having a sheet resistance of 12Ω/□) formed on one of thesurfaces of a glass substrate followed by drying at 130° C. andcalcination at 450° C. for one hour was repeated three times. A firstthin film containing randomly oriented zinc oxide needle-like crystalswas thus obtained. The film thickness of the first thin film was 4.7 μm.

Further, for the purpose of comparison, a thin film (second thin film)made of fine particles of zinc oxide was produced. A paste of fineparticles of zinc oxide was obtained by wet-kneading fine particles ofzinc oxide having an average particle diameter of 20 nm withacetylacetone, polyethylene glycol, and distilled water. An operation toapply this paste to a transparent conductive film (F-doped SnO₂ having asheet resistance of 12Ω/□) formed on one of the surfaces of a glasssubstrate followed by drying at 130° C. and calcination at 450° C. forone hour was repeated three times. A second thin film containingrandomly oriented fine particles of zinc oxide was thus obtained. Thefilm thickness of the second thin film was about 4.5 μm.

Dye sensitizing photovoltaic cells respectively using the array, thefirst thin film, and the second thin film were produced by the procedureas follows.

Initially, an ethanol solution in which was dissolved an Ru complex washeld at 40° C., and together with a glass substrate on which atransparent conductive film serving as the base was formed, each of thearray and the two kinds of thin films was dipped in the solution for 15minutes for an Ru dye to be absorbed in the surfaces of the array andeach thin film. The counter electrode to the transparent conductive film(the base of the array) used herein was prepared by vapor depositing1000-angstrom-thick platinum on a transparent conductive film (F-dopedSnO₂ having a sheet resistance of 12Ω/□) formed on one of the surfacesof a glass substrate.

By interposing a spacer between the counter electrode and the base, aminute space was defined therebetween and an electrolytic solution wasinjected into this minute space. A solution prepared by dissolvinglithium iodide and iodine in acetonitrile was used as the electrolyticsolution. Dye sensitizing photovoltaic cells were thus obtained.

The dye sensitizing photovoltaic cells fabricated in this manner werecompared under the conditions that the power generation area was abouttwo squares centimeter and an amount of absorbed dye was about 95nanomol/square centimeter. As a consequence, the conversion efficiencyper unit dye amount was the highest with the dye sensitizingphotovoltaic cell using the array (array of zinc oxide needle-likecrystals of the present invention) and the lowest with the one using thefirst thin film (the thin film containing randomly oriented zinc oxideneedle-like crystals).

In reference to an amount of power generation by the dye sensitizingphotovoltaic cell using the first thin film, an amount of powergeneration by the one using the array was 210% higher and the one usingthe second thin film was 80% higher.

The electrochemical properties were evaluated by measuring impedance ofthe array and the thin films (samples) of the two kinds. The measurementwas performed by means of triode measurement under irradiation of whitelight. Aluminum foil was connected to the sample to be measured and usedas a working electrode while a counter electrode made of platinum and areference electrode made of silver were used. As the electrolyte, anacetonitrile solution containing iodine at 0.05 mol/l and lithium iodideat 0.1 mol/l was used.

The measurement was made by the alternating-current impedance method.The subject was held for one minute under irradiation of light in astate where no voltage was applied and a stable voltage was used as theapplied voltage. The frequency range of the measurement was set to 100kHz to 100 mHz.

The results were shown in FIG. 15 in the form of the Bode diagram.Referring to FIG. 15, the curve g1 shows the measurement result of theoriented needle-like crystals (array), the curve g2 shows themeasurement result of the thin film (first thin film) made ofnon-oriented zinc oxide needle-like crystals, and the curve g3 shows themeasurement result of the thin film (second thin film) made of fineparticles of zinc oxide. It is understood that a response rate of theoriented needle-like crystals (array) is higher than those of the othersamples.

Example 11

The following will describe a case where a composite comprising a baseand an array of needle-like crystals of oxide formed on the surface ofthe base by means of electroless plating was produced.

A glass substrate (made of so-called soda glass) was prepared and usedas the base. A Kapton tape (registered trademark) was sticked on one ofthe surfaces of the base as a mask, and the face on which was stickedthis tape was defined as the back surface and the face on which wassticked no tape was defined as the surface.

Subsequently, the base was dipped in an electroless plating solutionwithin a reaction container (pressure-resistant container) made ofTeflon (registered trademark). The used electroless plating solution wasmade up of water as a solvent and zinc nitrate hexahydrate and sodiumhydroxide (NaOH) as a solute. The concentration of zinc nitratehexahydrate in the plating solution was 0.06 mol/l and the concentrationof sodium hydroxide in the plating solution was 0.75 mol/l. The pH ofthe plating solution was 13.2. When the plating solution was heated at85° C. for three hours within the reaction container, an array ofneedle-like crystals made of zinc oxide was formed on the surface of thebase.

As with Example 1, while the base was dipped in the plating solution(including the time of heating), the base was supported using asupporter in a posture for the surface of the base (the surface on theside where needle-like crystals were to be grown) to face the bottomsurface of the reaction container (faced downward). When configured inthis manner, the density of needle-like crystals grown in plural axialdirections from the tip ends of needle-like crystals that have beengrown directly from the base becomes lower, which makes it possible toobtain an array having an excellent light transmitting property.

When confirmed by an electron microscope, the base was substantiallycovered with the array of needle-like crystals in the resultingcomposite. The diameter of needle-like crystals was about 750 nm at thelargest and about 60 nm at the smallest. The length of needle-likecrystals was in the neighborhood of 3 μm. The density of needle-likecrystals in this instance was about 18 crystals per square μm.

In a case where the producing method using electrolytic plating insteadof electroless plating is adopted, the base is limited to a substancehaving the conducting property. It is therefore difficult to form anarray of needle-like crystals on the glass substrate as was in thisexample. In addition, in a case where an array of needle-like crystalsis formed on the base having a complicated shape, such as a designedarticle, it is difficult to control the electric field concentration inthe case of electrolytic plating. However, in the case of electrolessplating, a homogeneous array can be readily formed while the producingapparatus is left simple. Further, in a case where an array ofneedle-like crystals is formed on a large base, when only the solvent,the catalyst, and the temperature are placed under the same conditionsin essentials, the growing rate of needle-like crystals is almost thesame in every point of the base in the case of electroless plating. Anarray having a uniform thickness can be therefore readily obtained.

Even in a case where needle-like crystals are grown directly from theglass substrate as in this example, it is possible to achieve theadvantage of electroless plating (see Example 1) in comparison theelectrolytic plating.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross section of a photovoltaic conversion elementaccording to a first embodiment of the present invention.

FIG. 2 is a schematic cross section of a photovoltaic conversion elementaccording to a second embodiment of the present invention.

FIG. 3 is a schematic cross section of a photovoltaic conversion elementaccording to a third embodiment of the present invention.

FIG. 4 is a schematic cross section of a capacitor according to a firstembodiment of the present invention.

FIG. 5 is a schematic cross section of a capacitor according to a secondembodiment of the present invention.

FIG. 6 is a cross section showing a posture of a base within a reactioncontainer during electroless plating.

FIG. 7A shows scanning electron microscope images of an array of Example1.

FIG. 7B is a scanning electron microscope image of the array shown inFIG. 7A when it is separated from the base, which is taken from the sidewhere the base was present.

FIG. 8A is a scanning electron microscope image of an array of Example2.

FIG. 8B is a view showing the X-ray diffraction pattern of the arrayshown in FIG. 8A measured on a side (first region) where the base wasplaced and on the opposite side to the base (second region).

FIG. 9 is a scanning electron microscope image of an array (firstsample) of Example 3.

FIG. 10 is a scanning electron microscope image of another array (secondsample) of Example 3.

FIG. 11 is a view showing current versus voltage characteristics of abase exposed type composite in which the base is exposed through anarray of needle-like crystals and a base covered type composite in whichthe base is substantially covered with an array of needle-like crystals.

FIG. 12 is a view showing current versus voltage characteristics of dyesensitizing photovoltaic cells respectively using an array of zinc oxideneedle-like crystals through which the base is exposed in part and anarray of zinc oxide needle-like crystals substantially covering thebase.

FIG. 13A is a view showing an EBSP when an array of zinc oxideneedle-like crystals is observed from the first region side.

FIG. 13B is a view showing an EBSP when an array of zinc oxideneedle-like crystals is observed from the second region side.

FIG. 14A is a frequency distribution of plane orientations of zinc oxideneedle-like crystals on the first region side.

FIG. 14B is a frequency distribution of plane orientations of zinc oxideneedle-like crystals on the second region side.

FIG. 15 is a Bode diagram obtained by measuring impedance of an array ofneedle-like crystals, a thin film made of non-oriented zinc oxideneedle-like crystals, and a thin film made of particles of zinc oxide.

DESCRIPTION OF REFERENCE CHARACTERS

-   -   1, 1A, and 11: photovoltaic conversion element    -   2: transparent electrode    -   3, 23, and 33: needle-like crystals    -   3A, 23A, and 33A: first needle-like crystals    -   3B, 23B, and 33B: second needle-like crystals    -   3C: third needle-like crystals (external needle-like crystals)    -   4, 24, and 34: array    -   5: p-type semiconductor portion    -   7 and 27: microscopic region    -   10, 20, 30, and 40: composite    -   21 and 31: capacitor    -   22: storage node    -   25: protective film    -   32: substrate    -   35: electrolytic solution    -   123: plating solution    -   124: base    -   R1 and R21: first region    -   R2 and R22: second region    -   S: staring point

1. A composite comprising a base and an array of a plurality of needle-like crystals made of oxide and formed on a surface of the base: wherein the array includes a first region on a side of the base and a second region on an opposite side to the base with respect to the first region; and wherein a proportion of a cross section of the needle-like crystals in a plane parallel to the surface of the base is lower in the second region than in the first region and the surface of the base is substantially covered with the needle-like crystals in the first region.
 2. A composite comprising a base and an array of a plurality of needle-like crystals made of oxide and formed on a surface of the base: wherein the array includes a first region on a side of the base and a second region on an opposite side to the base with respect to the first region; and wherein an orientation degree of the needle-like crystals in the first region is low in comparison with the second region, and the surface of the base is substantially covered with the needle-like crystals in the first region.
 3. The composite according to claim 2, wherein: a proportion of a cross section of the needle-like crystals in a plane parallel to the surface of the base is lower in the second region than in the first region.
 4. A composite comprising a base and an array of a plurality of needle-like crystals made of oxide and formed on a surface of the base: wherein the needle-like crystals include first needle-like crystals extending from respective plural starting points positioned spaced apart from one another on the surface of the base in a direction producing an angle with the surface of the base such that falls within a specific angular range, and second needle-like crystals extending from the respective starting points in a direction within a wider angular range encompassing the specific angular range and shorter than the first needle-like crystals; and wherein a portion on the surface of the base exposed through the first needle-like crystals is substantially covered with the second needle-like crystals.
 5. The composite according to claim 1, wherein: the needle-like crystals include first needle-like crystals extending from respective plural starting points positioned spaced apart from one another on the surface of the base in a direction producing an angle with the surface of the base such that falls within a specific angular range, and second needle-like crystals extending from the respective starting points in a direction within a wider angular range encompassing the specific angular range and shorter than the first needle-like crystals; and a portion on the surface of the base exposed through the first needle-like crystals is substantially covered with the second needle-like crystals.
 6. The composite according to claim 1, wherein: a longitudinal direction and a direction of a c-axis coincide with each other in the needle-like crystals and the longitudinal direction of the needle-like crystals is oriented in a specific direction.
 7. The composite according to claim 1, further containing: a plurality of external needle-like crystals extending from end portions of the needle-like crystals on an opposite side to the base to an outside of an array region of the needle-like crystals.
 8. The composite according to claim 7, wherein: the plurality of external needle-like crystals extend in random directions within an angular range within which no interference with the array occurs.
 9. The composite according to claim 1, wherein: the needle-like crystals are made of zinc oxide.
 10. The composite according to claim 1, wherein: a plurality of microscopic regions having random crystal orientations are present in a vicinity of an interface between the array and the base.
 11. A photovoltaic conversion element comprising: the composite set forth in claim 1, wherein: the array in the composite is of one conduction type; and the photovoltaic conversion element further comprises a semiconductor portion of an opposite conduction type opposing a surface of the needle-like crystals.
 12. A light emitting element comprising: the composite set forth in claim 1, wherein: the array in the composite is of one conduction type; and the light emitting element further comprises a semiconductor portion of an opposite conduction type opposing a surface of the needle-like crystals.
 13. A capacitor comprising: the composite set forth in claim 1, wherein: the array in the composite functions as a first electrode; and the capacitor further comprises a second electrode opposing the first electrode and a dielectric material interposed between the first electrode and the second electrode.
 14. A capacitor comprising: the composite set forth in claim 1, wherein: the array in the composite functions as a first polarizable electrode; and the capacitor further comprises a second polarizable electrode opposing the first polarizable electrode and an electrolytic solution interposed between the first polarizable electrode and the second polarizable electrode.
 15. A method for producing the composite set forth in claim 1 comprising: a step of forming the needle-like crystals on a foundation made of a plurality of crystal grains that are crystal grains having random crystal orientations by means of electroless plating using a plating solution containing at least one kind of alkali selected from the group consisting of X¹OH, where X¹ is one of Na, K, and Cs, X² ₂CO₃, where X² is one of H, Na, K, and Cs, and NH₃ and having a pH of 13 or higher.
 16. A method for producing the composite set forth in claim 1 comprising: a step of forming the needle-like crystals on a foundation having a hydrophilic surface and being amorphous by means of electroless plating using a plating solution containing at least one kind of alkali selected from the group consisting of X¹OH, where X¹ is one of Na, K, and Cs, X² ₂CO₃, where X² is one of H, Na, K, and Cs, and NH₃ and having a pH of 13 or higher.
 17. The method for producing the composite according to claim 15, wherein: the foundation contains a surface portion of the base.
 18. The method for producing the composite according to claim 15, further comprising: a step of placing particles on the surface of the base, wherein the foundation contains the particles placed on the base.
 19. A method for producing a composite comprising a base and an array of a plurality of needle-like crystals made of oxide and formed on a surface of the base: wherein the array includes a first region on a side of the base and a second region on an opposite side to the base with respect to the first region; and wherein a proportion of a cross section of the needle-like crystals in a plane parallel to the surface of the base is lower in the second region than in the first region and the surface of the base is substantially covered with the needle-like crystals in the first region; and wherein a plurality of microscopic regions having random crystal orientations are present in a vicinity of an interface between the array and the base according to claim 18, wherein: the particles are made of a same material as the microscopic regions.
 20. The method for producing the composite according to claim 18, wherein: the step of placing the particles includes a step of forming a thin film made of the particles to have an average thickness of 50 nm or smaller on the base.
 21. The method for producing the composite according to claim 18, wherein: the step of placing the particles includes a step of forming a thin film made of the particles to have an average thickness of 20 nm or smaller on the base.
 22. The method for producing the composite according to claim 18, wherein the step of placing the particles includes: a step of placing a precursor of a substance forming the particles on the base; and a step of forming the particles by decomposing the precursor. 